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4.4. Logical Volume Administration

This section describes the commands that perform the various aspects of logical volume administration.

4.4.1. Creating Linear Logical Volumes

To create a logical volume, use the lvcreate command. If you do not specify a name for the logical volume, the default name lvol# is used where # is the internal number of the logical volume.
When you create a logical volume, the logical volume is carved from a volume group using the free extents on the physical volumes that make up the volume group. Normally logical volumes use up any space available on the underlying physical volumes on a next-free basis. Modifying the logical volume frees and reallocates space in the physical volumes.
The following command creates a logical volume 10 gigabytes in size in the volume group vg1.
# lvcreate -L 10G vg1
The default unit for logical volume size is megabytes. The following command creates a 1500 megabyte linear logical volume named testlv in the volume group testvg, creating the block device /dev/testvg/testlv.
# lvcreate -L 1500 -n testlv testvg
The following command creates a 50 gigabyte logical volume named gfslv from the free extents in volume group vg0.
# lvcreate -L 50G -n gfslv vg0
You can use the -l argument of the lvcreate command to specify the size of the logical volume in extents. You can also use this argument to specify the percentage of of the size of a related volume group, logical volume, or set of physical volumes. The suffix %VG denotes the total size of the volume group, the suffix %FREE the remaining free space in the volume group, and the suffix %PVS the free space in the specified physical volumes. For a snapshot, the size can be expressed as a percentage of the total size of the origin logical volume with the suffix %ORIGIN (100%ORIGIN provides space for the whole origin). When expressed as a percentage, the size defines an upper limit for the number of logical extents in the new logical volume. The precise number of logical extents in the new LV is not determined until the command has completed.
The following command creates a logical volume called mylv that uses 60% of the total space in volume group testvg.
# lvcreate -l 60%VG -n mylv testvg
The following command creates a logical volume called yourlv that uses all of the unallocated space in the volume group testvg.
# lvcreate -l 100%FREE -n yourlv testvg
You can use -l argument of the lvcreate command to create a logical volume that uses the entire volume group. Another way to create a logical volume that uses the entire volume group is to use the vgdisplay command to find the "Total PE" size and to use those results as input to the lvcreate command.
The following commands create a logical volume called mylv that fills the volume group named testvg.
# vgdisplay testvg | grep "Total PE"
Total PE              10230
# lvcreate -l 10230 -n mylv testvg
The underlying physical volumes used to create a logical volume can be important if the physical volume needs to be removed, so you may need to consider this possibility when you create the logical volume. For information on removing a physical volume from a volume group, see Section 4.3.7, “Removing Physical Volumes from a Volume Group”.
To create a logical volume to be allocated from a specific physical volume in the volume group, specify the physical volume or volumes at the end at the lvcreate command line. The following command creates a logical volume named testlv in volume group testvg allocated from the physical volume /dev/sdg1,
# lvcreate -L 1500 -n testlv testvg /dev/sdg1
You can specify which extents of a physical volume are to be used for a logical volume. The following example creates a linear logical volume out of extents 0 through 24 of physical volume /dev/sda1 and extents 50 through 124 of physical volume /dev/sdb1 in volume group testvg.
# lvcreate -l 100 -n testlv testvg /dev/sda1:0-24 /dev/sdb1:50-124
The following example creates a linear logical volume out of extents 0 through 25 of physical volume /dev/sda1 and then continues laying out the logical volume at extent 100.
# lvcreate -l 100 -n testlv testvg /dev/sda1:0-25:100-
The default policy for how the extents of a logical volume are allocated is inherit, which applies the same policy as for the volume group. These policies can be changed using the lvchange command. For information on allocation policies, see Section 4.3.1, “Creating Volume Groups”.

4.4.2. Creating Striped Volumes

For large sequential reads and writes, creating a striped logical volume can improve the efficiency of the data I/O. For general information about striped volumes, see Section 2.3.2, “Striped Logical Volumes”.
When you create a striped logical volume, you specify the number of stripes with the -i argument of the lvcreate command. This determines over how many physical volumes the logical volume will be striped. The number of stripes cannot be greater than the number of physical volumes in the volume group (unless the --alloc anywhere argument is used).
If the underlying physical devices that make up a striped logical volume are different sizes, the maximum size of the striped volume is determined by the smallest underlying device. For example, in a two-legged stripe, the maximum size is twice the size of the smaller device. In a three-legged stripe, the maximum size is three times the size of the smallest device.
The following command creates a striped logical volume across 2 physical volumes with a stripe of 64 kilobytes. The logical volume is 50 gigabytes in size, is named gfslv, and is carved out of volume group vg0.
# lvcreate -L 50G -i 2 -I 64 -n gfslv vg0
As with linear volumes, you can specify the extents of the physical volume that you are using for the stripe. The following command creates a striped volume 100 extents in size that stripes across two physical volumes, is named stripelv and is in volume group testvg. The stripe will use sectors 0-49 of /dev/sda1 and sectors 50-99 of /dev/sdb1.
# lvcreate -l 100 -i 2 -n stripelv testvg /dev/sda1:0-49 /dev/sdb1:50-99
  Using default stripesize 64.00 KB
  Logical volume "stripelv" created

4.4.3. RAID Logical Volumes

LVM supports RAID0/1/4/5/6/10.

Note

RAID logical volumes are not cluster-aware. While RAID logical volumes can be created and activated exclusively on one machine, they cannot be activated simultaneously on more than one machine. If you require non-exclusive mirrored volumes, you must create the volumes with a mirror segment type, as described in Section 4.4.4, “Creating Mirrored Volumes”.
To create a RAID logical volume, you specify a raid type as the --type argument of the lvcreate command. Table 4.1, “RAID Segment Types” describes the possible RAID segment types.

Table 4.1. RAID Segment Types

Segment typeDescription
raid1 RAID1 mirroring. This is the default value for the --type argument of the lvcreate command when you specify the -m but you do not specify striping.
raid4 RAID4 dedicated parity disk
raid5 Same as raid5_ls
raid5_la
RAID5 left asymmetric.
Rotating parity 0 with data continuation
raid5_ra
RAID5 right asymmetric.
Rotating parity N with data continuation
raid5_ls
RAID5 left symmetric.
Rotating parity 0 with data restart
raid5_rs
RAID5 right symmetric.
Rotating parity N with data restart
raid6 Same as raid6_zr
raid6_zr
RAID6 zero restart
Rotating parity zero (left-to-right) with data restart
raid6_nr
RAID6 N restart
Rotating parity N (left-to-right) with data restart
raid6_nc
RAID6 N continue
Rotating parity N (left-to-right) with data continuation
raid10
Striped mirrors. This is the default value for the --type argument of the lvcreate command if you specify the -m and you specify a number of stripes that is greater than 1.
Striping of mirror sets
raid0/raid0_meta (Red Hat Enterprise Linux 7.3 and later) Striping. RAID0 spreads logical volume data across multiple data subvolumes in units of stripe size. This is used to increase performance. Logical volume data will be lost if any of the data subvolumes fail. For information on creating RAID0 volumes, see Section 4.4.3.1, “Creating RAID0 Volumes (Red Hat Enterprise Linux 7.3 and Later)”.
For most users, specifying one of the five available primary types (raid1, raid4, raid5, raid6, raid10) should be sufficient.
When you create a RAID logical volume, LVM creates a metadata subvolume that is one extent in size for every data or parity subvolume in the array. For example, creating a 2-way RAID1 array results in two metadata subvolumes (lv_rmeta_0 and lv_rmeta_1) and two data subvolumes (lv_rimage_0 and lv_rimage_1). Similarly, creating a 3-way stripe (plus 1 implicit parity device) RAID4 results in 4 metadata subvolumes (lv_rmeta_0, lv_rmeta_1, lv_rmeta_2, and lv_rmeta_3) and 4 data subvolumes (lv_rimage_0, lv_rimage_1, lv_rimage_2, and lv_rimage_3).
The following command creates a 2-way RAID1 array named my_lv in the volume group my_vg that is one gigabyte in size.
# lvcreate --type raid1 -m 1 -L 1G -n my_lv my_vg
You can create RAID1 arrays with different numbers of copies according to the value you specify for the -m argument. Similarly, you specify the number of stripes for a RAID 4/5/6 logical volume with the -i argument. You can also specify the stripe size with the -I argument.
The following command creates a RAID5 array (3 stripes + 1 implicit parity drive) named my_lv in the volume group my_vg that is one gigabyte in size. Note that you specify the number of stripes just as you do for an LVM striped volume; the correct number of parity drives is added automatically.
# lvcreate --type raid5 -i 3 -L 1G -n my_lv my_vg
The following command creates a RAID6 array (3 stripes + 2 implicit parity drives) named my_lv in the volume group my_vg that is one gigabyte in size.
# lvcreate --type raid6 -i 3 -L 1G -n my_lv my_vg
After you have created a RAID logical volume with LVM, you can activate, change, remove, display, and use the volume just as you would any other LVM logical volume.
When you create RAID10 logical volumes, the background I/O required to initialize the logical volumes with a sync operation can crowd out other I/O operations to LVM devices, such as updates to volume group metadata, particularly when you are creating many RAID logical volumes. This can cause the other LVM operations to slow down.
You can control the rate at which a RAID logical volume is initialized by implementing recovery throttling. You control the rate at which sync operations are performed by setting the minimum and maximum I/O rate for those operations with the --minrecoveryrate and --maxrecoveryrate options of the lvcreate command. You specify these options as follows.
  • --maxrecoveryrate Rate[bBsSkKmMgG]
    Sets the maximum recovery rate for a RAID logical volume so that it will not crowd out nominal I/O operations. The Rate is specified as an amount per second for each device in the array. If no suffix is given, then kiB/sec/device is assumed. Setting the recovery rate to 0 means it will be unbounded.
  • --minrecoveryrate Rate[bBsSkKmMgG]
    Sets the minimum recovery rate for a RAID logical volume to ensure that I/O for sync operations achieves a minimum throughput, even when heavy nominal I/O is present. The Rate is specified as an amount per second for each device in the array. If no suffix is given, then kiB/sec/device is assumed.
The following command creates a 2-way RAID10 array with 3 stripes that is 10 gigabytes in size with a maximum recovery rate of 128 kiB/sec/device. The array is named my_lv and is in the volume group my_vg.
# lvcreate --type raid10 -i 2 -m 1 -L 10G --maxrecoveryrate 128 -n my_lv my_vg
You can also specify minimum and maximum recovery rates for a RAID scrubbing operation. For information on RAID scrubbing, see Section 4.4.3.11, “Scrubbing a RAID Logical Volume”.

Note

You can generate commands to create logical volumes on RAID storage with the LVM RAID Calculator application. This application uses the information you input about your current or planned storage to generate these commands. The LVM RAID Calculator application can be found at https://access.redhat.com/labs/lvmraidcalculator/.
The following sections describes the administrative tasks you can perform on LVM RAID devices:

4.4.3.1. Creating RAID0 Volumes (Red Hat Enterprise Linux 7.3 and Later)

The format for the command to create a RAID0 volume is as follows.
lvcreate --type raid0[_meta] --stripes Stripes --stripesize StripeSize VolumeGroup [PhysicalVolumePath ...]

Table 4.2. RAID0 Command Creation parameters

ParameterDescription
--type raid0[_meta] Specifying raid0 creates a RAID0 volume without metadata volumes. Specifying raid0_meta creates a RAID0 volume with metadata volumes. Because RAID0 is non-resilient, it does not have to store any mirrored data blocks as RAID1/10 or calculate and store any parity blocks as RAID4/5/6 do. Hence, it does not need metadata volumes to keep state about resynchronization progress of mirrored or parity blocks. Metadata volumes become mandatory on a conversion from RAID0 to RAID4/5/6/10, however, and specifying raid0_meta preallocates those metadata volumes to prevent a respective allocation failure.
--stripes Stripes Specifies the number of devices to spread the logical volume across.
--stripesize StripeSize Specifies the size of each stripe in kilobytes. This is the amount of data that is written to one device before moving to the next device.
VolumeGroup Specifies the volume group to use.
PhysicalVolumePath ... Specifies the devices to use. If this is not specified, LVM will choose the number of devices specified by the Stripes option, one for each stripe.

4.4.3.2. Converting a Linear Device to a RAID Device

You can convert an existing linear logical volume to a RAID device by using the --type argument of the lvconvert command.
The following command converts the linear logical volume my_lv in volume group my_vg to a 2-way RAID1 array.
# lvconvert --type raid1 -m 1 my_vg/my_lv
Since RAID logical volumes are composed of metadata and data subvolume pairs, when you convert a linear device to a RAID1 array, a new metadata subvolume is created and associated with the original logical volume on (one of) the same physical volumes that the linear volume is on. The additional images are added in metadata/data subvolume pairs. For example, if the original device is as follows:
# lvs -a -o name,copy_percent,devices my_vg
  LV     Copy%  Devices     
  my_lv         /dev/sde1(0)
After conversion to a 2-way RAID1 array the device contains the following data and metadata subvolume pairs:
# lvconvert --type raid1 -m 1 my_vg/my_lv
# lvs -a -o name,copy_percent,devices my_vg
  LV               Copy%  Devices                      
  my_lv            6.25   my_lv_rimage_0(0),my_lv_rimage_1(0)
  [my_lv_rimage_0]        /dev/sde1(0)                 
  [my_lv_rimage_1]        /dev/sdf1(1)                 
  [my_lv_rmeta_0]         /dev/sde1(256)               
  [my_lv_rmeta_1]         /dev/sdf1(0)
If the metadata image that pairs with the original logical volume cannot be placed on the same physical volume, the lvconvert will fail.

4.4.3.3. Converting an LVM RAID1 Logical Volume to an LVM Linear Logical Volume

You can convert an existing RAID1 LVM logical volume to an LVM linear logical volume with the lvconvert command by specifying the -m0 argument. This removes all the RAID data subvolumes and all the RAID metadata subvolumes that make up the RAID array, leaving the top-level RAID1 image as the linear logical volume.
The following example displays an existing LVM RAID1 logical volume.
# lvs -a -o name,copy_percent,devices my_vg
  LV               Copy%  Devices                      
  my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0)
  [my_lv_rimage_0]        /dev/sde1(1)                 
  [my_lv_rimage_1]        /dev/sdf1(1)                 
  [my_lv_rmeta_0]         /dev/sde1(0)                 
  [my_lv_rmeta_1]         /dev/sdf1(0)
The following command converts the LVM RAID1 logical volume my_vg/my_lv to an LVM linear device.
# lvconvert -m0 my_vg/my_lv
# lvs -a -o name,copy_percent,devices my_vg
  LV      Copy%  Devices     
  my_lv          /dev/sde1(1)
When you convert an LVM RAID1 logical volume to an LVM linear volume, you can specify which physical volumes to remove. The following example shows the layout of an LVM RAID1 logical volume made up of two images: /dev/sda1 and /dev/sdb1. In this example, the lvconvert command specifies that you want to remove /dev/sda1, leaving /dev/sdb1 as the physical volume that makes up the linear device.
# lvs -a -o name,copy_percent,devices my_vg
  LV               Copy%  Devices
  my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0)
  [my_lv_rimage_0]        /dev/sda1(1)
  [my_lv_rimage_1]        /dev/sdb1(1)
  [my_lv_rmeta_0]         /dev/sda1(0)
  [my_lv_rmeta_1]         /dev/sdb1(0)
# lvconvert -m0 my_vg/my_lv /dev/sda1
# lvs -a -o name,copy_percent,devices my_vg
  LV    Copy%  Devices
  my_lv        /dev/sdb1(1)

4.4.3.4. Converting a Mirrored LVM Device to a RAID1 Device

You can convert an existing mirrored LVM device with a segment type of mirror to a RAID1 LVM device with the lvconvert command by specifying the --type raid1 argument. This renames the mirror subvolumes (*_mimage_*) to RAID subvolumes (*_rimage_*). In addition, the mirror log is removed and metadata subvolumes (*_rmeta_*) are created for the data subvolumes on the same physical volumes as the corresponding data subvolumes.
The following example shows the layout of a mirrored logical volume my_vg/my_lv.
# lvs -a -o name,copy_percent,devices my_vg
  LV               Copy%  Devices                      
  my_lv             15.20 my_lv_mimage_0(0),my_lv_mimage_1(0)
  [my_lv_mimage_0]        /dev/sde1(0)                 
  [my_lv_mimage_1]        /dev/sdf1(0)                 
  [my_lv_mlog]            /dev/sdd1(0)
The following command converts the mirrored logical volume my_vg/my_lv to a RAID1 logical volume.
# lvconvert --type raid1 my_vg/my_lv
# lvs -a -o name,copy_percent,devices my_vg
  LV               Copy%  Devices                      
  my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0)
  [my_lv_rimage_0]        /dev/sde1(0)                 
  [my_lv_rimage_1]        /dev/sdf1(0)                 
  [my_lv_rmeta_0]         /dev/sde1(125)               
  [my_lv_rmeta_1]         /dev/sdf1(125)

4.4.3.5. Resizing a RAID Logical Volume

You can resize a RAID logical volume in the following ways;
  • You can increase the size of a RAID logical volume of any type with the lvresize or lvextend command. This does not change the number of RAID images. For striped RAID logical volumes the same stripe rounding constraints apply as when you create a striped RAID logical volume. For more information on extending a RAID volume, see Section 4.4.18, “Extending a RAID Volume”.
  • You can reduce the size of a RAID logical volume of any type with the lvresize or lvreduce command. This does not change the number of RAID images. As with the lvextend command, the same stripe rounding constraints apply as when you create a striped RAID logical volume. For an example of a command to reduce the size of a logical volume, see Section 4.4.16, “Shrinking Logical Volumes”.
  • As of Red Hat Enterprise Linux 7.4, you can change the number of stripes on a striped RAID logical volume (raid4/5/6/10) with the --stripes N parameter of the lvconvert command. This increases or reduces the size of the RAID logical volume by the capacity of the stripes added or removed. Note that raid10 volumes are capable only of adding stripes. This capability is part of the RAID reshaping feature that allows you to change attributes of a RAID logical volume while keeping the same RAID level. For information on RAID reshaping and examples of using the lvconvert command to reshape a RAID logical volume, see the lvmraid(7) man page.

4.4.3.6. Changing the Number of Images in an Existing RAID1 Device

You can change the number of images in an existing RAID1 array just as you can change the number of images in the earlier implementation of LVM mirroring. Use the lvconvert command to specify the number of additional metadata/data subvolume pairs to add or remove. For information on changing the volume configuration in the earlier implementation of LVM mirroring, see Section 4.4.4.4, “Changing Mirrored Volume Configuration”.
When you add images to a RAID1 device with the lvconvert command, you can specify the total number of images for the resulting device, or you can specify how many images to add to the device. You can also optionally specify on which physical volumes the new metadata/data image pairs will reside.
Metadata subvolumes (named *_rmeta_*) always exist on the same physical devices as their data subvolume counterparts *_rimage_*). The metadata/data subvolume pairs will not be created on the same physical volumes as those from another metadata/data subvolume pair in the RAID array (unless you specify --alloc anywhere).
The format for the command to add images to a RAID1 volume is as follows:
lvconvert -m new_absolute_count vg/lv [removable_PVs]
lvconvert -m +num_additional_images vg/lv [removable_PVs]
For example, the following command displays the LVM device my_vg/my_lv, which is a 2-way RAID1 array:
# lvs -a -o name,copy_percent,devices my_vg
  LV                Copy%  Devices                      
  my_lv             6.25    my_lv_rimage_0(0),my_lv_rimage_1(0)
  [my_lv_rimage_0]         /dev/sde1(0)                 
  [my_lv_rimage_1]         /dev/sdf1(1)                 
  [my_lv_rmeta_0]          /dev/sde1(256)               
  [my_lv_rmeta_1]          /dev/sdf1(0)
The following command converts the 2-way RAID1 device my_vg/my_lv to a 3-way RAID1 device:
# lvconvert -m 2 my_vg/my_lv
# lvs -a -o name,copy_percent,devices my_vg
  LV               Copy%  Devices                                     
  my_lv              6.25 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
  [my_lv_rimage_0]        /dev/sde1(0)                                
  [my_lv_rimage_1]        /dev/sdf1(1)                                
  [my_lv_rimage_2]        /dev/sdg1(1)                                
  [my_lv_rmeta_0]         /dev/sde1(256)                              
  [my_lv_rmeta_1]         /dev/sdf1(0)                                
  [my_lv_rmeta_2]         /dev/sdg1(0)
When you add an image to a RAID1 array, you can specify which physical volumes to use for the image. The following command converts the 2-way RAID1 device my_vg/my_lv to a 3-way RAID1 device, specifying that the physical volume /dev/sdd1 be used for the array:
# lvs -a -o name,copy_percent,devices my_vg
  LV               Copy%  Devices
  my_lv             56.00 my_lv_rimage_0(0),my_lv_rimage_1(0)
  [my_lv_rimage_0]        /dev/sda1(1)
  [my_lv_rimage_1]        /dev/sdb1(1)
  [my_lv_rmeta_0]         /dev/sda1(0)
  [my_lv_rmeta_1]         /dev/sdb1(0)
# lvconvert -m 2 my_vg/my_lv /dev/sdd1
# lvs -a -o name,copy_percent,devices my_vg
  LV               Copy%  Devices
  my_lv             28.00 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
  [my_lv_rimage_0]        /dev/sda1(1)
  [my_lv_rimage_1]        /dev/sdb1(1)
  [my_lv_rimage_2]        /dev/sdd1(1)
  [my_lv_rmeta_0]         /dev/sda1(0)
  [my_lv_rmeta_1]         /dev/sdb1(0)
  [my_lv_rmeta_2]         /dev/sdd1(0)
To remove images from a RAID1 array, use the following command. When you remove images from a RAID1 device with the lvconvert command, you can specify the total number of images for the resulting device, or you can specify how many images to remove from the device. You can also optionally specify the physical volumes from which to remove the device.
lvconvert -m new_absolute_count vg/lv [removable_PVs]
lvconvert -m -num_fewer_images vg/lv [removable_PVs]
Additionally, when an image and its associated metadata subvolume volume are removed, any higher-numbered images will be shifted down to fill the slot. If you remove lv_rimage_1 from a 3-way RAID1 array that consists of lv_rimage_0, lv_rimage_1, and lv_rimage_2, this results in a RAID1 array that consists of lv_rimage_0 and lv_rimage_1. The subvolume lv_rimage_2 will be renamed and take over the empty slot, becoming lv_rimage_1.
The following example shows the layout of a 3-way RAID1 logical volume my_vg/my_lv.
# lvs -a -o name,copy_percent,devices my_vg
  LV               Copy%  Devices                                     
  my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
  [my_lv_rimage_0]        /dev/sde1(1)                                
  [my_lv_rimage_1]        /dev/sdf1(1)                                
  [my_lv_rimage_2]        /dev/sdg1(1)                                
  [my_lv_rmeta_0]         /dev/sde1(0)                                
  [my_lv_rmeta_1]         /dev/sdf1(0)                                
  [my_lv_rmeta_2]         /dev/sdg1(0)
The following command converts the 3-way RAID1 logical volume into a 2-way RAID1 logical volume.
# lvconvert -m1 my_vg/my_lv
# lvs -a -o name,copy_percent,devices my_vg
  LV               Copy%  Devices                      
  my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0)
  [my_lv_rimage_0]        /dev/sde1(1)                 
  [my_lv_rimage_1]        /dev/sdf1(1)                 
  [my_lv_rmeta_0]         /dev/sde1(0)                 
  [my_lv_rmeta_1]         /dev/sdf1(0)
The following command converts the 3-way RAID1 logical volume into a 2-way RAID1 logical volume, specifying the physical volume that contains the image to remove as /dev/sde1.
# lvconvert -m1 my_vg/my_lv /dev/sde1
# lvs -a -o name,copy_percent,devices my_vg
  LV               Copy%  Devices                      
  my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0)
  [my_lv_rimage_0]        /dev/sdf1(1)                 
  [my_lv_rimage_1]        /dev/sdg1(1)                 
  [my_lv_rmeta_0]         /dev/sdf1(0)                 
  [my_lv_rmeta_1]         /dev/sdg1(0)

4.4.3.7. Splitting off a RAID Image as a Separate Logical Volume

You can split off an image of a RAID logical volume to form a new logical volume. The procedure for splitting off a RAID image is the same as the procedure for splitting off a redundant image of a mirrored logical volume, as described in Section 4.4.4.2, “Splitting Off a Redundant Image of a Mirrored Logical Volume”.
The format of the command to split off a RAID image is as follows:
lvconvert --splitmirrors count -n splitname vg/lv [removable_PVs]
Just as when you are removing a RAID image from an existing RAID1 logical volume (as described in Section 4.4.3.6, “Changing the Number of Images in an Existing RAID1 Device”), when you remove a RAID data subvolume (and its associated metadata subvolume) from the middle of the device any higher numbered images will be shifted down to fill the slot. The index numbers on the logical volumes that make up a RAID array will thus be an unbroken sequence of integers.

Note

You cannot split off a RAID image if the RAID1 array is not yet in sync.
The following example splits a 2-way RAID1 logical volume, my_lv, into two linear logical volumes, my_lv and new.
# lvs -a -o name,copy_percent,devices my_vg
  LV               Copy%  Devices                      
  my_lv             12.00 my_lv_rimage_0(0),my_lv_rimage_1(0)
  [my_lv_rimage_0]        /dev/sde1(1)                 
  [my_lv_rimage_1]        /dev/sdf1(1)                 
  [my_lv_rmeta_0]         /dev/sde1(0)                 
  [my_lv_rmeta_1]         /dev/sdf1(0)                 
# lvconvert --splitmirror 1 -n new my_vg/my_lv
# lvs -a -o name,copy_percent,devices my_vg
  LV      Copy%  Devices     
  my_lv          /dev/sde1(1)
  new            /dev/sdf1(1)
The following example splits a 3-way RAID1 logical volume, my_lv, into a 2-way RAID1 logical volume, my_lv, and a linear logical volume, new
# lvs -a -o name,copy_percent,devices my_vg
  LV               Copy%  Devices                                     
  my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
  [my_lv_rimage_0]        /dev/sde1(1)                                
  [my_lv_rimage_1]        /dev/sdf1(1)                                
  [my_lv_rimage_2]        /dev/sdg1(1)                                
  [my_lv_rmeta_0]         /dev/sde1(0)                                
  [my_lv_rmeta_1]         /dev/sdf1(0)                                
  [my_lv_rmeta_2]         /dev/sdg1(0)                                
# lvconvert --splitmirror 1 -n new my_vg/my_lv
# lvs -a -o name,copy_percent,devices my_vg
  LV            Copy%  Devices                      
  my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0)
  [my_lv_rimage_0]        /dev/sde1(1)                 
  [my_lv_rimage_1]        /dev/sdf1(1)                 
  [my_lv_rmeta_0]         /dev/sde1(0)                 
  [my_lv_rmeta_1]         /dev/sdf1(0)                 
  new                     /dev/sdg1(1)

4.4.3.8. Splitting and Merging a RAID Image

You can temporarily split off an image of a RAID1 array for read-only use while keeping track of any changes by using the --trackchanges argument in conjunction with the --splitmirrors argument of the lvconvert command. This allows you to merge the image back into the array at a later time while resyncing only those portions of the array that have changed since the image was split.
The format for the lvconvert command to split off a RAID image is as follows.
lvconvert --splitmirrors count --trackchanges vg/lv [removable_PVs]
When you split off a RAID image with the --trackchanges argument, you can specify which image to split but you cannot change the name of the volume being split. In addition, the resulting volumes have the following constraints.
  • The new volume you create is read-only.
  • You cannot resize the new volume.
  • You cannot rename the remaining array.
  • You cannot resize the remaining array.
  • You can activate the new volume and the remaining array independently.
You can merge an image that was split off with the --trackchanges argument specified by executing a subsequent lvconvert command with the --merge argument. When you merge the image, only the portions of the array that have changed since the image was split are resynced.
The format for the lvconvert command to merge a RAID image is as follows.
lvconvert --merge raid_image
The following example creates a RAID1 logical volume and then splits off an image from that volume while tracking changes to the remaining array.
# lvcreate --type raid1 -m 2 -L 1G -n my_lv .vg
  Logical volume "my_lv" created
# lvs -a -o name,copy_percent,devices my_vg
  LV               Copy%  Devices                                     
  my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
  [my_lv_rimage_0]        /dev/sdb1(1)                                
  [my_lv_rimage_1]        /dev/sdc1(1)                                
  [my_lv_rimage_2]        /dev/sdd1(1)                                
  [my_lv_rmeta_0]         /dev/sdb1(0)                                
  [my_lv_rmeta_1]         /dev/sdc1(0)                                
  [my_lv_rmeta_2]         /dev/sdd1(0)                                
# lvconvert --splitmirrors 1 --trackchanges my_vg/my_lv
  my_lv_rimage_2 split from my_lv for read-only purposes.
  Use 'lvconvert --merge my_vg/my_lv_rimage_2' to merge back into my_lv
# lvs -a -o name,copy_percent,devices my_vg
  LV               Copy%  Devices                                     
  my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
  [my_lv_rimage_0]        /dev/sdb1(1)                                
  [my_lv_rimage_1]        /dev/sdc1(1)                                
  my_lv_rimage_2          /dev/sdd1(1)                                
  [my_lv_rmeta_0]         /dev/sdb1(0)                                
  [my_lv_rmeta_1]         /dev/sdc1(0)                                
  [my_lv_rmeta_2]         /dev/sdd1(0)
The following example splits off an image from a RAID1 volume while tracking changes to the remaining array, then merges the volume back into the array.
# lvconvert --splitmirrors 1 --trackchanges my_vg/my_lv
  lv_rimage_1 split from my_lv for read-only purposes.
  Use 'lvconvert --merge my_vg/my_lv_rimage_1' to merge back into my_lv
# lvs -a -o name,copy_percent,devices my_vg
  LV               Copy%  Devices                      
  my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0)
  [my_lv_rimage_0]        /dev/sdc1(1)                 
  my_lv_rimage_1          /dev/sdd1(1)                 
  [my_lv_rmeta_0]         /dev/sdc1(0)                 
  [my_lv_rmeta_1]         /dev/sdd1(0)                 
# lvconvert --merge my_vg/my_lv_rimage_1
  my_vg/my_lv_rimage_1 successfully merged back into my_vg/my_lv
# lvs -a -o name,copy_percent,devices my_vg
  LV               Copy%  Devices                      
  my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0)
  [my_lv_rimage_0]        /dev/sdc1(1)                 
  [my_lv_rimage_1]        /dev/sdd1(1)                 
  [my_lv_rmeta_0]         /dev/sdc1(0)                 
  [my_lv_rmeta_1]         /dev/sdd1(0)
Once you have split off an image from a RAID1 volume, you can make the split permanent by issuing a second lvconvert --splitmirrors command, repeating the initial lvconvert command that split the image without specifying the --trackchanges argument. This breaks the link that the --trackchanges argument created.
After you have split an image with the --trackchanges argument, you cannot issue a subsequent lvconvert --splitmirrors command on that array unless your intent is to permanently split the image being tracked.
The following sequence of commands splits an image and tracks the image and then permanently splits off the image being tracked.
# lvconvert --splitmirrors 1 --trackchanges my_vg/my_lv
  my_lv_rimage_1 split from my_lv for read-only purposes.
  Use 'lvconvert --merge my_vg/my_lv_rimage_1' to merge back into my_lv
# lvconvert --splitmirrors 1 -n new my_vg/my_lv
# lvs -a -o name,copy_percent,devices my_vg
  LV   Copy%  Devices     
  my_lv          /dev/sdc1(1)
  new            /dev/sdd1(1)
Note, however, that the following sequence of commands will fail.
# lvconvert --splitmirrors 1 --trackchanges my_vg/my_lv
  my_lv_rimage_1 split from my_lv for read-only purposes.
  Use 'lvconvert --merge my_vg/my_lv_rimage_1' to merge back into my_lv
# lvconvert --splitmirrors 1 --trackchanges my_vg/my_lv
  Cannot track more than one split image at a time
Similarly, the following sequence of commands will fail as well, since the split image is not the image being tracked.
# lvconvert --splitmirrors 1 --trackchanges my_vg/my_lv
  my_lv_rimage_1 split from my_lv for read-only purposes.
  Use 'lvconvert --merge my_vg/my_lv_rimage_1' to merge back into my_lv
# lvs -a -o name,copy_percent,devices my_vg
  LV               Copy%  Devices                      
  my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0)
  [my_lv_rimage_0]        /dev/sdc1(1)                 
  my_lv_rimage_1          /dev/sdd1(1)                 
  [my_lv_rmeta_0]         /dev/sdc1(0)                 
  [my_lv_rmeta_1]         /dev/sdd1(0)                 
# lvconvert --splitmirrors 1 -n new my_vg/my_lv /dev/sdc1
  Unable to split additional image from my_lv while tracking changes for my_lv_rimage_1

4.4.3.9. Setting a RAID fault policy

LVM RAID handles device failures in an automatic fashion based on the preferences defined by the raid_fault_policy field in the lvm.conf file.
  • If the raid_fault_policy field is set to allocate, the system will attempt to replace the failed device with a spare device from the volume group. If there is no available spare device, this will be reported to the system log.
  • If the raid_fault_policy field is set to warn, the system will produce a warning and the log will indicate that a device has failed. This allows the user to determine the course of action to take.
As long as there are enough devices remaining to support usability, the RAID logical volume will continue to operate.
4.4.3.9.1. The allocate RAID Fault Policy
In the following example, the raid_fault_policy field has been set to allocate in the lvm.conf file. The RAID logical volume is laid out as follows.
# lvs -a -o name,copy_percent,devices my_vg
  LV               Copy%  Devices                                     
  my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
  [my_lv_rimage_0]        /dev/sde1(1)                                
  [my_lv_rimage_1]        /dev/sdf1(1)                                
  [my_lv_rimage_2]        /dev/sdg1(1)                                
  [my_lv_rmeta_0]         /dev/sde1(0)                                
  [my_lv_rmeta_1]         /dev/sdf1(0)                                
  [my_lv_rmeta_2]         /dev/sdg1(0)
If the /dev/sde device fails, the system log will display error messages.
# grep lvm /var/log/messages 
Jan 17 15:57:18 bp-01 lvm[8599]: Device #0 of raid1 array, my_vg-my_lv, has failed.
Jan 17 15:57:18 bp-01 lvm[8599]: /dev/sde1: read failed after 0 of 2048 at
250994294784: Input/output error
Jan 17 15:57:18 bp-01 lvm[8599]: /dev/sde1: read failed after 0 of 2048 at
250994376704: Input/output error
Jan 17 15:57:18 bp-01 lvm[8599]: /dev/sde1: read failed after 0 of 2048 at 0:
Input/output error
Jan 17 15:57:18 bp-01 lvm[8599]: /dev/sde1: read failed after 0 of 2048 at
4096: Input/output error
Jan 17 15:57:19 bp-01 lvm[8599]: Couldn't find device with uuid
3lugiV-3eSP-AFAR-sdrP-H20O-wM2M-qdMANy.
Jan 17 15:57:27 bp-01 lvm[8599]: raid1 array, my_vg-my_lv, is not in-sync.
Jan 17 15:57:36 bp-01 lvm[8599]: raid1 array, my_vg-my_lv, is now in-sync.
Since the raid_fault_policy field has been set to allocate, the failed device is replaced with a new device from the volume group.
# lvs -a -o name,copy_percent,devices vg
  Couldn't find device with uuid 3lugiV-3eSP-AFAR-sdrP-H20O-wM2M-qdMANy.
  LV            Copy%  Devices                                     
  lv            100.00 lv_rimage_0(0),lv_rimage_1(0),lv_rimage_2(0)
  [lv_rimage_0]        /dev/sdh1(1)                                
  [lv_rimage_1]        /dev/sdf1(1)                                
  [lv_rimage_2]        /dev/sdg1(1)                                
  [lv_rmeta_0]         /dev/sdh1(0)                                
  [lv_rmeta_1]         /dev/sdf1(0)                                
  [lv_rmeta_2]         /dev/sdg1(0)
Note that even though the failed device has been replaced, the display still indicates that LVM could not find the failed device. This is because, although the failed device has been removed from the RAID logical volume, the failed device has not yet been removed from the volume group. To remove the failed device from the volume group, you can execute vgreduce --removemissing VG.
If the raid_fault_policy has been set to allocate but there are no spare devices, the allocation will fail, leaving the logical volume as it is. If the allocation fails, you have the option of fixing the drive, then deactivating and activating the logical volume; this is described in Section 4.4.3.9.2, “The warn RAID Fault Policy”. Alternately, you can replace the failed device, as described in Section 4.4.3.10, “Replacing a RAID device”.
4.4.3.9.2. The warn RAID Fault Policy
In the following example, the raid_fault_policy field has been set to warn in the lvm.conf file. The RAID logical volume is laid out as follows.
# lvs -a -o name,copy_percent,devices my_vg
  LV               Copy%  Devices                                     
  my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
  [my_lv_rimage_0]        /dev/sdh1(1)                                
  [my_lv_rimage_1]        /dev/sdf1(1)                                
  [my_lv_rimage_2]        /dev/sdg1(1)                                
  [my_lv_rmeta_0]         /dev/sdh1(0)                                
  [my_lv_rmeta_1]         /dev/sdf1(0)                                
  [my_lv_rmeta_2]         /dev/sdg1(0)
If the /dev/sdh device fails, the system log will display error messages. In this case, however, LVM will not automatically attempt to repair the RAID device by replacing one of the images. Instead, if the device has failed you can replace the device with the --repair argument of the lvconvert command, as shown below.
# lvconvert --repair my_vg/my_lv
  /dev/sdh1: read failed after 0 of 2048 at 250994294784: Input/output error
  /dev/sdh1: read failed after 0 of 2048 at 250994376704: Input/output error
  /dev/sdh1: read failed after 0 of 2048 at 0: Input/output error
  /dev/sdh1: read failed after 0 of 2048 at 4096: Input/output error
  Couldn't find device with uuid fbI0YO-GX7x-firU-Vy5o-vzwx-vAKZ-feRxfF.
Attempt to replace failed RAID images (requires full device resync)? [y/n]: y

# lvs -a -o name,copy_percent,devices my_vg
  Couldn't find device with uuid fbI0YO-GX7x-firU-Vy5o-vzwx-vAKZ-feRxfF.
  LV               Copy%  Devices                                     
  my_lv             64.00 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
  [my_lv_rimage_0]        /dev/sde1(1)                                
  [my_lv_rimage_1]        /dev/sdf1(1)                                
  [my_lv_rimage_2]        /dev/sdg1(1)                                
  [my_lv_rmeta_0]         /dev/sde1(0)                                
  [my_lv_rmeta_1]         /dev/sdf1(0)                                
  [my_lv_rmeta_2]         /dev/sdg1(0)
Note that even though the failed device has been replaced, the display still indicates that LVM could not find the failed device. This is because, although the failed device has been removed from the RAID logical volume, the failed device has not yet been removed from the volume group. To remove the failed device from the volume group, you can execute vgreduce --removemissing VG.
If the device failure is a transient failure or you are able to repair the device that failed, you can initiate recovery of the failed device with the --refresh option of the lvchange command. Previously it was necessary to deactivate and then activate the logical volume.
The following command refreshes a logical volume.
# lvchange --refresh my_vg/my_lv

4.4.3.10. Replacing a RAID device

RAID is not like traditional LVM mirroring. LVM mirroring required failed devices to be removed or the mirrored logical volume would hang. RAID arrays can keep on running with failed devices. In fact, for RAID types other than RAID1, removing a device would mean converting to a lower level RAID (for example, from RAID6 to RAID5, or from RAID4 or RAID5 to RAID0). Therefore, rather than removing a failed device unconditionally and potentially allocating a replacement, LVM allows you to replace a device in a RAID volume in a one-step solution by using the --replace argument of the lvconvert command.
The format for the lvconvert --replace is as follows.
lvconvert --replace dev_to_remove vg/lv [possible_replacements]
The following example creates a RAID1 logical volume and then replaces a device in that volume.
# lvcreate --type raid1 -m 2 -L 1G -n my_lv my_vg
  Logical volume "my_lv" created
# lvs -a -o name,copy_percent,devices my_vg
  LV               Copy%  Devices                                     
  my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
  [my_lv_rimage_0]        /dev/sdb1(1)                                
  [my_lv_rimage_1]        /dev/sdb2(1)                                
  [my_lv_rimage_2]        /dev/sdc1(1)                                
  [my_lv_rmeta_0]         /dev/sdb1(0)                                
  [my_lv_rmeta_1]         /dev/sdb2(0)                                
  [my_lv_rmeta_2]         /dev/sdc1(0)                                
# lvconvert --replace /dev/sdb2 my_vg/my_lv
# lvs -a -o name,copy_percent,devices my_vg
  LV               Copy%  Devices                                     
  my_lv             37.50 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
  [my_lv_rimage_0]        /dev/sdb1(1)                                
  [my_lv_rimage_1]        /dev/sdc2(1)                                
  [my_lv_rimage_2]        /dev/sdc1(1)                                
  [my_lv_rmeta_0]         /dev/sdb1(0)                                
  [my_lv_rmeta_1]         /dev/sdc2(0)                                
  [my_lv_rmeta_2]         /dev/sdc1(0)
The following example creates a RAID1 logical volume and then replaces a device in that volume, specifying which physical volume to use for the replacement.
# lvcreate --type raid1 -m 1 -L 100 -n my_lv my_vg
  Logical volume "my_lv" created
# lvs -a -o name,copy_percent,devices my_vg
  LV               Copy%  Devices
  my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0)
  [my_lv_rimage_0]        /dev/sda1(1)
  [my_lv_rimage_1]        /dev/sdb1(1)
  [my_lv_rmeta_0]         /dev/sda1(0)
  [my_lv_rmeta_1]         /dev/sdb1(0)
# pvs
  PV          VG       Fmt  Attr PSize    PFree
  /dev/sda1   my_vg    lvm2 a--  1020.00m  916.00m
  /dev/sdb1   my_vg    lvm2 a--  1020.00m  916.00m
  /dev/sdc1   my_vg    lvm2 a--  1020.00m 1020.00m
  /dev/sdd1   my_vg    lvm2 a--  1020.00m 1020.00m
# lvconvert --replace /dev/sdb1 my_vg/my_lv /dev/sdd1
# lvs -a -o name,copy_percent,devices my_vg
  LV               Copy%  Devices
  my_lv             28.00 my_lv_rimage_0(0),my_lv_rimage_1(0)
  [my_lv_rimage_0]        /dev/sda1(1)
  [my_lv_rimage_1]        /dev/sdd1(1)
  [my_lv_rmeta_0]         /dev/sda1(0)
  [my_lv_rmeta_1]         /dev/sdd1(0)
You can replace more than one RAID device at a time by specifying multiple replace arguments, as in the following example.
# lvcreate --type raid1 -m 2 -L 100 -n my_lv my_vg
  Logical volume "my_lv" created
# lvs -a -o name,copy_percent,devices my_vg
  LV               Copy%  Devices
  my_lv            100.00 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
  [my_lv_rimage_0]        /dev/sda1(1)
  [my_lv_rimage_1]        /dev/sdb1(1)
  [my_lv_rimage_2]        /dev/sdc1(1)
  [my_lv_rmeta_0]         /dev/sda1(0)
  [my_lv_rmeta_1]         /dev/sdb1(0)
  [my_lv_rmeta_2]         /dev/sdc1(0)
# lvconvert --replace /dev/sdb1 --replace /dev/sdc1 my_vg/my_lv
# lvs -a -o name,copy_percent,devices my_vg
  LV               Copy%  Devices
  my_lv             60.00 my_lv_rimage_0(0),my_lv_rimage_1(0),my_lv_rimage_2(0)
  [my_lv_rimage_0]        /dev/sda1(1)
  [my_lv_rimage_1]        /dev/sdd1(1)
  [my_lv_rimage_2]        /dev/sde1(1)
  [my_lv_rmeta_0]         /dev/sda1(0)
  [my_lv_rmeta_1]         /dev/sdd1(0)
  [my_lv_rmeta_2]         /dev/sde1(0)

Note

When you specify a replacement drive using the lvconvert --replace command, the replacement drives should never be allocated from extra space on drives already used in the array. For example, lv_rimage_0 and lv_rimage_1 should not be located on the same physical volume.

4.4.3.11. Scrubbing a RAID Logical Volume

LVM provides scrubbing support for RAID logical volumes. RAID scrubbing is the process of reading all the data and parity blocks in an array and checking to see whether they are coherent.
You initiate a RAID scrubbing operation with the --syncaction option of the lvchange command. You specify either a check or repair operation. A check operation goes over the array and records the number of discrepancies in the array but does not repair them. A repair operation corrects the discrepancies as it finds them.
The format of the command to scrub a RAID logical volume is as follows:
lvchange --syncaction {check|repair} vg/raid_lv

Note

The lvchange --syncaction repair vg/raid_lv operation does not perform the same function as the lvconvert --repair vg/raid_lv operation. The lvchange --syncaction repair operation initiates a background synchronization operation on the array, while the lvconvert --repair operation is designed to repair/replace failed devices in a mirror or RAID logical volume.
In support of the new RAID scrubbing operation, the lvs command now supports two new printable fields: raid_sync_action and raid_mismatch_count. These fields are not printed by default. To display these fields you specify them with the -o parameter of the lvs, as follows.
lvs -o +raid_sync_action,raid_mismatch_count vg/lv
The raid_sync_action field displays the current synchronization operation that the raid volume is performing. It can be one of the following values:
  • idle: All sync operations complete (doing nothing)
  • resync: Initializing an array or recovering after a machine failure
  • recover: Replacing a device in the array
  • check: Looking for array inconsistencies
  • repair: Looking for and repairing inconsistencies
The raid_mismatch_count field displays the number of discrepancies found during a check operation.
The Cpy%Sync field of the lvs command now prints the progress of any of the raid_sync_action operations, including check and repair.
The lv_attr field of the lvs command output now provides additional indicators in support of the RAID scrubbing operation. Bit 9 of this field displays the health of the logical volume, and it now supports the following indicators.
  • (m)ismatches indicates that there are discrepancies in a RAID logical volume. This character is shown after a scrubbing operation has detected that portions of the RAID are not coherent.
  • (r)efresh indicates that a device in a RAID array has suffered a failure and the kernel regards it as failed, even though LVM can read the device label and considers the device to be operational. The logical volume should be (r)efreshed to notify the kernel that the device is now available, or the device should be (r)eplaced if it is suspected of having failed.
For information on the lvs command, see Section 4.8.2, “Object Display Fields”.
When you perform a RAID scrubbing operation, the background I/O required by the sync operations can crowd out other I/O operations to LVM devices, such as updates to volume group metadata. This can cause the other LVM operations to slow down. You can control the rate at which the RAID logical volume is scrubbed by implementing recovery throttling.
You control the rate at which sync operations are performed by setting the minimum and maximum I/O rate for those operations with the --minrecoveryrate and --maxrecoveryrate options of the lvchange command. You specify these options as follows.
  • --maxrecoveryrate Rate[bBsSkKmMgG]
    Sets the maximum recovery rate for a RAID logical volume so that it will not crowd out nominal I/O operations. The Rate is specified as an amount per second for each device in the array. If no suffix is given, then kiB/sec/device is assumed. Setting the recovery rate to 0 means it will be unbounded.
  • --minrecoveryrate Rate[bBsSkKmMgG]
    Sets the minimum recovery rate for a RAID logical volume to ensure that I/O for sync operations achieves a minimum throughput, even when heavy nominal I/O is present. The Rate is specified as an amount per second for each device in the array. If no suffix is given, then kiB/sec/device is assumed.

4.4.3.12. RAID Takeover (Red Hat Enterprise Linux 7.4 and Later)

LVM supports Raid takeover, which means converting a RAID logical volume from one RAID level to another (such as from RAID 5 to RAID 6). Changing the RAID level is usually done to increase or decrease resilience to device failures or to restripe logical volumes. You use the lvconvert for RAID takeover. For information on RAID takeover and for examples of using the lvconvert to convert a RAID logical volume, see the lvmraid(7) man page.

4.4.3.13. Reshaping a RAID Logical Volume (Red Hat Enterprise Linux 7.4 and Later)

RAID reshaping means changing attributes of a RAID logical volume while keeping the same RAID level. Some attributes you can change include RAID layout, stripe size, and number of stripes. For information on RAID reshaping and examples of using the lvconvert command to reshape a RAID logical volume, see the lvmraid(7) man page.

4.4.3.14. Controlling I/O Operations on a RAID1 Logical Volume

You can control the I/O operations for a device in a RAID1 logical volume by using the --writemostly and --writebehind parameters of the lvchange command. The format for using these parameters is as follows.
  • --[raid]writemostly PhysicalVolume[:{t|y|n}]
    Marks a device in a RAID1 logical volume as write-mostly. All reads to these drives will be avoided unless necessary. Setting this parameter keeps the number of I/O operations to the drive to a minimum. By default, the write-mostly attribute is set to yes for the specified physical volume in the logical volume. It is possible to remove the write-mostly flag by appending :n to the physical volume or to toggle the value by specifying :t. The --writemostly argument can be specified more than one time in a single command, making it possible to toggle the write-mostly attributes for all the physical volumes in a logical volume at once.
  • --[raid]writebehind IOCount
    Specifies the maximum number of outstanding writes that are allowed to devices in a RAID1 logical volume that are marked as write-mostly. Once this value is exceeded, writes become synchronous, causing all writes to the constituent devices to complete before the array signals the write has completed. Setting the value to zero clears the preference and allows the system to choose the value arbitrarily.

4.4.3.15. Changing the region size on a RAID Logical Volume (Red Hat Enterprise Linux 7.4 and later)

When you create a RAID logical volume, the region size for the logical volume will be the value of the raid_region_size parameter in the /etc/lvm/lvm.conf file. You can override this default value with the -R option of the lvcreate command.
After you have created a RAID logical volume, you can change the region size of the volume with the -R option of the lvconvert command. The following example changes the region size of logical volume vg/raidlv to 4096K. The RAID volume must be synced in order to change the region size.
# lvconvert -R 4096K vg/raid1
Do you really want to change the region_size 512.00 KiB of LV vg/raid1 to 4.00 MiB? [y/n]: y
  Changed region size on RAID LV vg/raid1 to 4.00 MiB.

4.4.4. Creating Mirrored Volumes

For the Red Hat Enterprise Linux 7.0 release, LVM supports RAID 1/4/5/6/10, as described in Section 4.4.3, “RAID Logical Volumes”. RAID logical volumes are not cluster-aware. While RAID logical volumes can be created and activated exclusively on one machine, they cannot be activated simultaneously on more than one machine. If you require non-exclusive mirrored volumes, you must create the volumes with a mirror segment type, as described in this section.

Note

For information on converting an existing LVM device with a segment type of mirror to a RAID1 LVM device, see Section 4.4.3.4, “Converting a Mirrored LVM Device to a RAID1 Device”.

Note

Creating a mirrored LVM logical volume in a cluster requires the same commands and procedures as creating a mirrored LVM logical volume with a segment type of mirror on a single node. However, in order to create a mirrored LVM volume in a cluster, the cluster and cluster mirror infrastructure must be running, the cluster must be quorate, and the locking type in the lvm.conf file must be set correctly to enable cluster locking. For an example of creating a mirrored volume in a cluster, see Section 5.5, “Creating a Mirrored LVM Logical Volume in a Cluster”.
Attempting to run multiple LVM mirror creation and conversion commands in quick succession from multiple nodes in a cluster might cause a backlog of these commands. This might cause some of the requested operations to time out and, subsequently, fail. To avoid this issue, it is recommended that cluster mirror creation commands be executed from one node of the cluster.
When you create a mirrored volume, you specify the number of copies of the data to make with the -m argument of the lvcreate command. Specifying -m1 creates one mirror, which yields two copies of the file system: a linear logical volume plus one copy. Similarly, specifying -m2 creates two mirrors, yielding three copies of the file system.
The following command creates a mirrored logical volume with a single mirror. The volume is 50 gigabytes in size, is named mirrorlv, and is carved out of volume group vg0:
# lvcreate --type mirror -L 50G -m 1 -n mirrorlv vg0
An LVM mirror divides the device being copied into regions that, by default, are 512KB in size. You can use the -R argument of the lvcreate command to specify the region size in megabytes. You can also change the default region size by editing the mirror_region_size setting in the lvm.conf file.

Note

Due to limitations in the cluster infrastructure, cluster mirrors greater than 1.5TB cannot be created with the default region size of 512KB. Users that require larger mirrors should increase the region size from its default to something larger. Failure to increase the region size will cause LVM creation to hang and may hang other LVM commands as well.
As a general guideline for specifying the region size for mirrors that are larger than 1.5TB, you could take your mirror size in terabytes and round up that number to the next power of 2, using that number as the -R argument to the lvcreate command. For example, if your mirror size is 1.5TB, you could specify -R 2. If your mirror size is 3TB, you could specify -R 4. For a mirror size of 5TB, you could specify -R 8.
The following command creates a mirrored logical volume with a region size of 2MB:
# lvcreate --type mirror -m 1 -L 2T -R 2 -n mirror vol_group
When a mirror is created, the mirror regions are synchronized. For large mirror components, the sync process may take a long time. When you are creating a new mirror that does not need to be revived, you can specify the --nosync argument to indicate that an initial synchronization from the first device is not required.
LVM maintains a small log which it uses to keep track of which regions are in sync with the mirror or mirrors. By default, this log is kept on disk, which keeps it persistent across reboots and ensures that the mirror does not need to be re-synced every time a machine reboots or crashes. You can specify instead that this log be kept in memory with the --mirrorlog core argument; this eliminates the need for an extra log device, but it requires that the entire mirror be resynchronized at every reboot.
The following command creates a mirrored logical volume from the volume group bigvg. The logical volume is named ondiskmirvol and has a single mirror. The volume is 12MB in size and keeps the mirror log in memory.
# lvcreate --type mirror -L 12MB -m 1 --mirrorlog core -n ondiskmirvol bigvg
  Logical volume "ondiskmirvol" created
The mirror log is created on a separate device from the devices on which any of the mirror legs are created. It is possible, however, to create the mirror log on the same device as one of the mirror legs by using the --alloc anywhere argument of the vgcreate command. This may degrade performance, but it allows you to create a mirror even if you have only two underlying devices.
The following command creates a mirrored logical volume with a single mirror for which the mirror log is on the same device as one of the mirror legs. In this example, the volume group vg0 consists of only two devices. This command creates a 500 MB volume named mirrorlv in the vg0 volume group.
# lvcreate --type mirror -L 500M -m 1 -n mirrorlv -alloc anywhere vg0

Note

With clustered mirrors, the mirror log management is completely the responsibility of the cluster node with the currently lowest cluster ID. Therefore, when the device holding the cluster mirror log becomes unavailable on a subset of the cluster, the clustered mirror can continue operating without any impact, as long as the cluster node with lowest ID retains access to the mirror log. Since the mirror is undisturbed, no automatic corrective action (repair) is issued, either. When the lowest-ID cluster node loses access to the mirror log, however, automatic action will kick in (regardless of accessibility of the log from other nodes).
To create a mirror log that is itself mirrored, you can specify the --mirrorlog mirrored argument. The following command creates a mirrored logical volume from the volume group bigvg. The logical volume is named twologvol and has a single mirror. The volume is 12MB in size and the mirror log is mirrored, with each log kept on a separate device.
# lvcreate --type mirror -L 12MB -m 1 --mirrorlog mirrored -n twologvol bigvg
  Logical volume "twologvol" created
Just as with a standard mirror log, it is possible to create the redundant mirror logs on the same device as the mirror legs by using the --alloc anywhere argument of the vgcreate command. This may degrade performance, but it allows you to create a redundant mirror log even if you do not have sufficient underlying devices for each log to be kept on a separate device than the mirror legs.
When a mirror is created, the mirror regions are synchronized. For large mirror components, the sync process may take a long time. When you are creating a new mirror that does not need to be revived, you can specify the --nosync argument to indicate that an initial synchronization from the first device is not required.
You can specify which devices to use for the mirror legs and log, and which extents of the devices to use. To force the log onto a particular disk, specify exactly one extent on the disk on which it will be placed. LVM does not necessary respect the order in which devices are listed in the command line. If any physical volumes are listed that is the only space on which allocation will take place. Any physical extents included in the list that are already allocated will get ignored.
The following command creates a mirrored logical volume with a single mirror and a single log that is not mirrored. The volume is 500 MB in size, it is named mirrorlv, and it is carved out of volume group vg0. The first leg of the mirror is on device /dev/sda1, the second leg of the mirror is on device /dev/sdb1, and the mirror log is on /dev/sdc1.
# lvcreate --type mirror -L 500M -m 1 -n mirrorlv vg0 /dev/sda1 /dev/sdb1 /dev/sdc1
The following command creates a mirrored logical volume with a single mirror. The volume is 500 MB in size, it is named mirrorlv, and it is carved out of volume group vg0. The first leg of the mirror is on extents 0 through 499 of device /dev/sda1, the second leg of the mirror is on extents 0 through 499 of device /dev/sdb1, and the mirror log starts on extent 0 of device /dev/sdc1. These are 1MB extents. If any of the specified extents have already been allocated, they will be ignored.
# lvcreate --type mirror -L 500M -m 1 -n mirrorlv vg0 /dev/sda1:0-499 /dev/sdb1:0-499 /dev/sdc1:0

Note

You can combine striping and mirroring in a single logical volume. Creating a logical volume while simultaneously specifying the number of mirrors (--mirrors X) and the number of stripes (--stripes Y) results in a mirror device whose constituent devices are striped.

4.4.4.1. Mirrored Logical Volume Failure Policy

You can define how a mirrored logical volume behaves in the event of a device failure with the mirror_image_fault_policy and mirror_log_fault_policy parameters in the activation section of the lvm.conf file. When these parameters are set to remove, the system attempts to remove the faulty device and run without it. When these parameters are set to allocate, the system attempts to remove the faulty device and tries to allocate space on a new device to be a replacement for the failed device. This policy acts like the remove policy if no suitable device and space can be allocated for the replacement.
By default, the mirror_log_fault_policy parameter is set to allocate. Using this policy for the log is fast and maintains the ability to remember the sync state through crashes and reboots. If you set this policy to remove, when a log device fails the mirror converts to using an in-memory log; in this instance, the mirror will not remember its sync status across crashes and reboots and the entire mirror will be re-synced.
By default, the mirror_image_fault_policy parameter is set to remove. With this policy, if a mirror image fails the mirror will convert to a non-mirrored device if there is only one remaining good copy. Setting this policy to allocate for a mirror device requires the mirror to resynchronize the devices; this is a slow process, but it preserves the mirror characteristic of the device.

Note

When an LVM mirror suffers a device failure, a two-stage recovery takes place. The first stage involves removing the failed devices. This can result in the mirror being reduced to a linear device. The second stage, if the mirror_log_fault_policy parameter is set to allocate, is to attempt to replace any of the failed devices. Note, however, that there is no guarantee that the second stage will choose devices previously in-use by the mirror that had not been part of the failure if others are available.
For information on manually recovering from an LVM mirror failure, see Section 6.3, “Recovering from LVM Mirror Failure”.

4.4.4.2. Splitting Off a Redundant Image of a Mirrored Logical Volume

You can split off a redundant image of a mirrored logical volume to form a new logical volume. To split off an image, use the --splitmirrors argument of the lvconvert command, specifying the number of redundant images to split off. You must use the --name argument of the command to specify a name for the newly-split-off logical volume.
The following command splits off a new logical volume named copy from the mirrored logical volume vg/lv. The new logical volume contains two mirror legs. In this example, LVM selects which devices to split off.
# lvconvert --splitmirrors 2 --name copy vg/lv
You can specify which devices to split off. The following command splits off a new logical volume named copy from the mirrored logical volume vg/lv. The new logical volume contains two mirror legs consisting of devices /dev/sdc1 and /dev/sde1.
# lvconvert --splitmirrors 2 --name copy vg/lv /dev/sd[ce]1

4.4.4.3. Repairing a Mirrored Logical Device

You can use the lvconvert --repair command to repair a mirror after a disk failure. This brings the mirror back into a consistent state. The lvconvert --repair command is an interactive command that prompts you to indicate whether you want the system to attempt to replace any failed devices.
  • To skip the prompts and replace all of the failed devices, specify the -y option on the command line.
  • To skip the prompts and replace none of the failed devices, specify the -f option on the command line.
  • To skip the prompts and still indicate different replacement policies for the mirror image and the mirror log, you can specify the --use-policies argument to use the device replacement policies specified by the mirror_log_fault_policy and mirror_device_fault_policy parameters in the lvm.conf file.

4.4.4.4. Changing Mirrored Volume Configuration

You can increase or decrease the number of mirrors that a logical volume contains by using the lvconvert command. This allows you to convert a logical volume from a mirrored volume to a linear volume or from a linear volume to a mirrored volume. You can also use this command to reconfigure other mirror parameters of an existing logical volume, such as corelog.
When you convert a linear volume to a mirrored volume, you are creating mirror legs for an existing volume. This means that your volume group must contain the devices and space for the mirror legs and for the mirror log.
If you lose a leg of a mirror, LVM converts the volume to a linear volume so that you still have access to the volume, without the mirror redundancy. After you replace the leg, use the lvconvert command to restore the mirror. This procedure is provided in Section 6.3, “Recovering from LVM Mirror Failure”.
The following command converts the linear logical volume vg00/lvol1 to a mirrored logical volume.
# lvconvert -m1 vg00/lvol1
The following command converts the mirrored logical volume vg00/lvol1 to a linear logical volume, removing the mirror leg.
# lvconvert -m0 vg00/lvol1
The following example adds an additional mirror leg to the existing logical volume vg00/lvol1. This example shows the configuration of the volume before and after the lvconvert command changed the volume to a volume with two mirror legs.
# lvs -a -o name,copy_percent,devices vg00
  LV                  Copy%  Devices
  lvol1               100.00 lvol1_mimage_0(0),lvol1_mimage_1(0)
  [lvol1_mimage_0]        /dev/sda1(0)
  [lvol1_mimage_1]        /dev/sdb1(0)
  [lvol1_mlog]            /dev/sdd1(0)
# lvconvert -m 2 vg00/lvol1
  vg00/lvol1: Converted: 13.0%
  vg00/lvol1: Converted: 100.0%
  Logical volume lvol1 converted.
# lvs -a -o name,copy_percent,devices vg00
  LV                  Copy%  Devices
  lvol1               100.00 lvol1_mimage_0(0),lvol1_mimage_1(0),lvol1_mimage_2(0)
  [lvol1_mimage_0]        /dev/sda1(0)
  [lvol1_mimage_1]        /dev/sdb1(0)
  [lvol1_mimage_2]        /dev/sdc1(0)
  [lvol1_mlog]            /dev/sdd1(0)

4.4.5. Creating Thinly-Provisioned Logical Volumes

Logical volumes can be thinly provisioned. This allows you to create logical volumes that are larger than the available extents. Using thin provisioning, you can manage a storage pool of free space, known as a thin pool, which can be allocated to an arbitrary number of devices when needed by applications. You can then create devices that can be bound to the thin pool for later allocation when an application actually writes to the logical volume. The thin pool can be expanded dynamically when needed for cost-effective allocation of storage space.

Note

This section provides an overview of the basic commands you use to create and grow thinly-provisioned logical volumes. For detailed information on LVM thin provisioning as well as information on using the LVM commands and utilities with thinly-provisioned logical volumes, see the lvmthin(7) man page.

Note

Thin volumes are not supported across the nodes in a cluster. The thin pool and all its thin volumes must be exclusively activated on only one cluster node.
To create a thin volume, perform the following tasks:
  1. Create a volume group with the vgcreate command.
  2. Create a thin pool with the lvcreate command.
  3. Create a thin volume in the thin pool with the lvcreate command.
You can use the -T (or --thin) option of the lvcreate command to create either a thin pool or a thin volume. You can also use -T option of the lvcreate command to create both a thin pool and a thin volume in that pool at the same time with a single command.
The following command uses the -T option of the lvcreate command to create a thin pool named mythinpool in the volume group vg001 and that is 100M in size. Note that since you are creating a pool of physical space, you must specify the size of the pool. The -T option of the lvcreate command does not take an argument; it deduces what type of device is to be created from the other options the command specifies.
# lvcreate -L 100M -T vg001/mythinpool
  Rounding up size to full physical extent 4.00 MiB
  Logical volume "mythinpool" created
# lvs
  LV            VG     Attr     LSize   Pool Origin Data%  Move Log Copy% Convert
  my mythinpool vg001  twi-a-tz 100.00m               0.00
The following command uses the -T option of the lvcreate command to create a thin volume named thinvolume in the thin pool vg001/mythinpool. Note that in this case you are specifying virtual size, and that you are specifying a virtual size for the volume that is greater than the pool that contains it.
# lvcreate -V 1G -T vg001/mythinpool -n thinvolume
  Logical volume "thinvolume" created
# lvs
  LV          VG       Attr     LSize   Pool       Origin Data%  Move Log Copy%  Convert
  mythinpool  vg001    twi-a-tz 100.00m                     0.00                        
  thinvolume  vg001    Vwi-a-tz   1.00g mythinpool          0.00
The following command uses the -T option of the lvcreate command to create a thin pool and a thin volume in that pool by specifying both a size and a virtual size argument for the lvcreate command. This command creates a thin pool named mythinpool in the volume group vg001 and it also creates a thin volume named thinvolume in that pool.
# lvcreate -L 100M -T vg001/mythinpool -V 1G -n thinvolume
  Rounding up size to full physical extent 4.00 MiB
  Logical volume "thinvolume" created
# lvs
  LV           VG       Attr     LSize   Pool     Origin Data%  Move Log Copy%  Convert
  mythinpool   vg001    twi-a-tz 100.00m                   0.00                        
  thinvolume   vg001    Vwi-a-tz   1.00g mythinpool        0.00
You can also create a thin pool by specifying the --thinpool parameter of the lvcreate command. Unlike the -T option, the --thinpool parameter requires an argument, which is the name of the thin pool logical volume that you are creating. The following example specifies the --thinpool parameter of the lvcreate command to create a thin pool named mythinpool in the volume group vg001 and that is 100M in size:
# lvcreate -L 100M --thinpool mythinpool vg001
  Rounding up size to full physical extent 4.00 MiB
  Logical volume "mythinpool" created
# lvs
  LV          VG     Attr     LSize   Pool Origin Data%  Move Log Copy% Convert
  mythinpool  vg001  twi-a-tz 100.00m               0.00
Striping is supported for pool creation. The following command creates a 100M thin pool named pool in volume group vg001 with two 64 kB stripes and a chunk size of 256 kB. It also creates a 1T thin volume, vg00/thin_lv.
# lvcreate -i 2 -I 64 -c 256 -L 100M -T vg00/pool -V 1T --name thin_lv
You can extend the size of a thin volume with the lvextend command. You cannot, however, reduce the size of a thin pool.
The following command resizes an existing thin pool that is 100M in size by extending it another 100M.
# lvextend -L+100M vg001/mythinpool
  Extending logical volume mythinpool to 200.00 MiB
  Logical volume mythinpool successfully resized
# lvs
  LV           VG       Attr     LSize   Pool     Origin Data%  Move Log Copy%  Convert
  mythinpool   vg001    twi-a-tz 200.00m                   0.00                        
  thinvolume   vg001    Vwi-a-tz   1.00g mythinpool          0.00
As with other types of logical volumes, you can rename the volume with the lvrename, you can remove the volume with the lvremove, and you can display information about the volume with the lvs and lvdisplay commands.
By default, the lvcreate command sets the size of the thin pool's metadata logical volume according to the formula (Pool_LV_size / Pool_LV_chunk_size * 64). If you will have large numbers of snapshots or if you have small chunk sizes for your thin pool and thus expect significant growth of the size of the thin pool at a later time, you may need to increase the default value of the thin pool's metadata volume with the --poolmetadatasize parameter of the lvcreate command. The supported value for the thin pool's metadata logical volume is in the range between 2MiB and 16GiB.
You can use the --thinpool parameter of the lvconvert command to convert an existing logical volume to a thin pool volume. When you convert an existing logical volume to a thin pool volume, you must use the --poolmetadata parameter in conjunction with the --thinpool parameter of the lvconvert to convert an existing logical volume to the thin pool volume's metadata volume.

Note

Converting a logical volume to a thin pool volume or a thin pool metadata volume destroys the content of the logical volume, since in this case the lvconvert does not preserve the content of the devices but instead overwrites the content.
The following example converts the existing logical volume lv1 in volume group vg001 to a thin pool volume and converts the existing logical volume lv2 in volume group vg001 to the metadata volume for that thin pool volume.
# lvconvert --thinpool vg001/lv1 --poolmetadata vg001/lv2
  Converted vg001/lv1 to thin pool.

4.4.6. Creating Snapshot Volumes

Note

LVM supports thinly-provisioned snapshots. For information on creating thinly-provisioned snapshot volumes, see Section 4.4.7, “Creating Thinly-Provisioned Snapshot Volumes”.
Use the -s argument of the lvcreate command to create a snapshot volume. A snapshot volume is writable.

Note

LVM snapshots are not supported across the nodes in a cluster. You cannot create a snapshot volume in a clustered volume group. However, if you need to create a consistent backup of data on a clustered logical volume you can activate the volume exclusively and then create the snapshot. For information on activating logical volumes exclusively on one node, see Section 4.7, “Activating Logical Volumes on Individual Nodes in a Cluster”.

Note

LVM snapshots are supported for mirrored logical volumes.
Snapshots are supported for RAID logical volumes. For information on creating RAID logical volumes, see Section 4.4.3, “RAID Logical Volumes”.
LVM does not allow you to create a snapshot volume that is larger than the size of the origin volume plus needed metadata for the volume. If you specify a snapshot volume that is larger than this, the system will create a snapshot volume that is only as large as will be needed for the size of the origin.
By default, a snapshot volume is skipped during normal activation commands. For information on controlling the activation of a snapshot volume, see Section 4.4.20, “Controlling Logical Volume Activation”.
The following command creates a snapshot logical volume that is 100 MB in size named /dev/vg00/snap. This creates a snapshot of the origin logical volume named /dev/vg00/lvol1. If the original logical volume contains a file system, you can mount the snapshot logical volume on an arbitrary directory in order to access the contents of the file system to run a backup while the original file system continues to get updated.
# lvcreate --size 100M --snapshot --name snap /dev/vg00/lvol1
After you create a snapshot logical volume, specifying the origin volume on the lvdisplay command yields output that includes a list of all snapshot logical volumes and their status (active or inactive).
The following example shows the status of the logical volume /dev/new_vg/lvol0, for which a snapshot volume /dev/new_vg/newvgsnap has been created.
# lvdisplay /dev/new_vg/lvol0
  --- Logical volume ---
  LV Name                /dev/new_vg/lvol0
  VG Name                new_vg
  LV UUID                LBy1Tz-sr23-OjsI-LT03-nHLC-y8XW-EhCl78
  LV Write Access        read/write
  LV snapshot status     source of
                         /dev/new_vg/newvgsnap1 [active]
  LV Status              available
  # open                 0
  LV Size                52.00 MB
  Current LE             13
  Segments               1
  Allocation             inherit
  Read ahead sectors     0
  Block device           253:2
The lvs command, by default, displays the origin volume and the current percentage of the snapshot volume being used. The following example shows the default output for the lvs command for a system that includes the logical volume /dev/new_vg/lvol0, for which a snapshot volume /dev/new_vg/newvgsnap has been created.
# lvs
  LV         VG     Attr   LSize  Origin Snap%  Move Log Copy%
  lvol0      new_vg owi-a- 52.00M
  newvgsnap1 new_vg swi-a-  8.00M lvol0    0.20

Warning

Because the snapshot increases in size as the origin volume changes, it is important to monitor the percentage of the snapshot volume regularly with the lvs command to be sure it does not fill. A snapshot that is 100% full is lost completely, as a write to unchanged parts of the origin would be unable to succeed without corrupting the snapshot.
In addition to the snapshot itself being invalidated when full, any mounted file systems on that snapshot device are forcibly unmounted, avoiding the inevitable file system errors upon access to the mount point. In addition, you can specify the snapshot_autoextend_threshold option in the lvm.conf file. This option allows automatic extension of a snapshot whenever the remaining snapshot space drops below the threshold you set. This feature requires that there be unallocated space in the volume group.
LVM does not allow you to create a snapshot volume that is larger than the size of the origin volume plus needed metadata for the volume. Similarly, automatic extension of a snapshot will not increase the size of a snapshot volume beyond the maximum calculated size that is necessary for the snapshot. Once a snapshot has grown large enough to cover the origin, it is no longer monitored for automatic extension.
Information on setting snapshot_autoextend_threshold and snapshot_autoextend_percent is provided in the lvm.conf file itself. For information about the lvm.conf file, see Appendix B, The LVM Configuration Files.

4.4.7. Creating Thinly-Provisioned Snapshot Volumes

Red Hat Enterprise Linux provides support for thinly-provisioned snapshot volumes. For information on the benefits and limitations of thin snapshot volumes, see Section 2.3.6, “Thinly-Provisioned Snapshot Volumes”.

Note

This section provides an overview of the basic commands you use to create and grow thinly-provisioned snapshot volumes. For detailed information on LVM thin provisioning as well as information on using the LVM commands and utilities with thinly-provisioned logical volumes, see the lvmthin(7) man page.

Important

When creating a thin snapshot volume, you do not specify the size of the volume. If you specify a size parameter, the snapshot that will be created will not be a thin snapshot volume and will not use the thin pool for storing data. For example, the command lvcreate -s vg/thinvolume -L10M will not create a thin snapshot, even though the origin volume is a thin volume.
Thin snapshots can be created for thinly-provisioned origin volumes, or for origin volumes that are not thinly-provisioned.
You can specify a name for the snapshot volume with the --name option of the lvcreate command. The following command creates a thinly-provisioned snapshot volume of the thinly-provisioned logical volume vg001/thinvolume that is named mysnapshot1.
# lvcreate -s --name mysnapshot1 vg001/thinvolume
  Logical volume "mysnapshot1" created
# lvs
  LV          VG       Attr     LSize   Pool       Origin     Data%  Move Log Copy%  Convert
  mysnapshot1 vg001    Vwi-a-tz   1.00g mythinpool thinvolume   0.00                        
  mythinpool  vg001    twi-a-tz 100.00m                         0.00                        
  thinvolume  vg001    Vwi-a-tz   1.00g mythinpool              0.00
A thin snapshot volume has the same characteristics as any other thin volume. You can independently activate the volume, extend the volume, rename the volume, remove the volume, and even snapshot the volume.
By default, a snapshot volume is skipped during normal activation commands. For information on controlling the activation of a snapshot volume, see Section 4.4.20, “Controlling Logical Volume Activation”.
You can also create a thinly-provisioned snapshot of a non-thinly-provisioned logical volume. Since the non-thinly-provisioned logical volume is not contained within a thin pool, it is referred to as an external origin. External origin volumes can be used and shared by many thinly-provisioned snapshot volumes, even from different thin pools. The external origin must be inactive and read-only at the time the thinly-provisioned snapshot is created.
To create a thinly-provisioned snapshot of an external origin, you must specify the --thinpool option. The following command creates a thin snapshot volume of the read-only inactive volume origin_volume. The thin snapshot volume is named mythinsnap. The logical volume origin_volume then becomes the thin external origin for the thin snapshot volume mythinsnap in volume group vg001 that will use the existing thin pool vg001/pool. Because the origin volume must be in the same volume group as the snapshot volume, you do not need to specify the volume group when specifying the origin logical volume.
# lvcreate -s --thinpool vg001/pool origin_volume --name mythinsnap
You can create a second thinly-provisioned snapshot volume of the first snapshot volume, as in the following command.
# lvcreate -s vg001/mythinsnap --name my2ndthinsnap
As of Red Hat Enterprise Linux 7.2, you can display a list of all ancestors and descendants of a thin snapshot logical volume by specifying the lv_ancestors and lv_descendants reporting fields of the lvs command.
In the following example:
  • stack1 is an origin volume in volume group vg001.
  • stack2 is a snapshot of stack1
  • stack3 is a snapshot of stack2
  • stack4 is a snapshot of stack3
Additionally:
  • stack5 is also a snapshot of stack2
  • stack6 is a snapshot of stack5
$ lvs -o name,lv_ancestors,lv_descendants vg001
  LV      Ancestors              Descendants                  
  stack1                         stack2,stack3,stack4,stack5,stack6
  stack2  stack1                 stack3,stack4,stack5,stack6
  stack3  stack2,stack1          stack4                        
  stack4  stack3,stack2,stack1                              
  stack5  stack2,stack1          stack6
  stack6  stack5,stack2,stack1 
  pool

Note

The lv_ancestors and lv_descendants fields display existing dependencies but do not track removed entries which can break a dependency chain if the entry was removed from the middle of the chain. For example, if you remove the logical volume stack3 from this sample configuration, the display is as follows.
$ lvs -o name,lv_ancestors,lv_descendants vg001
  LV      Ancestors              Descendants
  stack1                         stack2,stack5,stack6
  stack2  stack1                 stack5,stack6
  stack4  
  stack5  stack2,stack1          stack6
  stack6  stack5,stack2,stack1
  pool
As of Red Hat Enterprise Linux 7.3, however, you can configure your system to track and display logical volumes that have been removed, and you can display the full dependency chain that includes those volumes by specifying the lv_ancestors_full and lv_descendants_full fields. For information on tracking, displaying, and removing historical logical volumes, see Section 4.4.21, “Tracking and Displaying Historical Logical Volumes (Red Hat Enterprise Linux 7.3 and Later)”.

4.4.8. Creating LVM Cache Logical Volumes

As of the Red Hat Enterprise Linux 7.1 release, LVM provides full support for LVM cache logical volumes. A cache logical volume uses a small logical volume consisting of fast block devices (such as SSD drives) to improve the performance of a larger and slower logical volume by storing the frequently used blocks on the smaller, faster logical volume.
LVM caching uses the following LVM logical volume types. All of these associated logical volumes must be in the same volume group.
  • Origin logical volume — the large, slow logical volume
  • Cache pool logical volume — the small, fast logical volume, which is composed of two devices: the cache data logical volume, and the cache metadata logical volume
  • Cache data logical volume — the logical volume containing the data blocks for the cache pool logical volume
  • Cache metadata logical volume — the logical volume containing the metadata for the cache pool logical volume, which holds the accounting information that specifies where data blocks are stored (for example, on the origin logical volume or the cache data logical volume).
  • Cache logical volume — the logical volume containing the origin logical volume and the cache pool logical volume. This is the resultant usable device which encapsulates the various cache volume components.
The following procedure creates an LVM cache logical volume.
  1. Create a volume group that contains a slow physical volume and a fast physical volume. In this example. /dev/sde1 is a slow device and /dev/sdf1 is a fast device and both devices are contained in volume group VG.
    # pvcreate /dev/sde1
    # pvcreate /dev/sdf1
    # vgcreate VG /dev/sde1 /dev/sdf1
  2. Create the origin volume. This example creates an origin volume named lv that is ten gigabytes in size and that consists of /dev/sde1, the slow physical volume.
    # lvcreate -L 10G -n lv VG /dev/sde1
  3. Create the cache pool logical volume. This example creates the cache pool logical volume named cpool on the fast device /dev/sdf1, which is part of the volume group VG. The cache pool logical volume this command creates consists of the hidden cache data logical volume cpool_cdata and the hidden cache metadata logical volume cpool_cmeta.
    # lvcreate --type cache-pool -L 5G -n cpool VG /dev/sdf1
      Using default stripesize 64.00 KiB.
      Logical volume "cpool" created.
    # lvs -a -o name,size,attr,devices VG
      LV              LSize  Attr       Devices
      [cpool]          5.00g Cwi---C--- cpool_cdata(0)
      [cpool_cdata]    5.00g Cwi-ao---- /dev/sdf1(4)
      [cpool_cmeta]    8.00m ewi-ao---- /dev/sdf1(2)
    
    For more complicated configurations you may need to create the cache data and the cache metadata logical volumes individually and then combine the volumes into a cache pool logical volume. For information on this procedure, see the lvmcache(7) man page.
  4. Create the cache logical volume by linking the cache pool logical volume to the origin logical volume. The resulting user-accessible cache logical volume takes the name of the origin logical volume. The origin logical volume becomes a hidden logical volume with _corig appended to the original name.
    # lvconvert --type cache --cachepool VG/lv cpool
      Logical volume cpool is now cached.
    # lvs -a -o name,size,attr,devices vg
        LV              LSize  Attr       Devices
        [cpool]          5.00g Cwi---C--- cpool_cdata(0)
        [cpool_cdata]    5.00g Cwi-ao---- /dev/sdf1(4)
        [cpool_cmeta]    8.00m ewi-ao---- /dev/sdf1(2)
        lv              10.00g Cwi-a-C--- lv_corig(0)
        [lv_corig]      10.00g owi-aoC--- /dev/sde1(0)
        [lvol0_pmspare]  8.00m ewi------- /dev/sdf1(0)
    
  5. Optionally, as of Red Hat Enterprise Linux release 7.2, you can convert the cached logical volume to a thin pool logical volume. Note that any thin logical volumes created from the pool will share the cache.
    The following command uses the fast device, /dev/sdf1, for allocating the thin pool metadata (lv_tmeta). This is the same device that is used by the cache pool volume, which means that the thin pool metadata volume shares that device with both the cache data logical volume cpool_cdata and the cache metadata logical volume cpool_cmeta.
    # lvconvert --type thin-pool VG/lv /dev/sdf1
       WARNING: Converting logical volume VG/lv to thin pool's data volume with metadata wiping.
       THIS WILL DESTROY CONTENT OF LOGICAL VOLUME (filesystem etc.)
     Do you really want to convert VG/lv? [y/n]: y
       Converted VG/lv to thin pool.
    # lvs -a -o name,size,attr,devices vg
       LV               LSize  Attr       Devices
       [cpool]           5.00g Cwi---C--- cpool_cdata(0)
       [cpool_cdata]     5.00g Cwi-ao---- /dev/sdf1(4)
       [cpool_cmeta]     8.00m ewi-ao---- /dev/sdf1(2)
       lv               10.00g twi-a-tz-- lv_tdata(0)
       [lv_tdata]       10.00g Cwi-aoC--- lv_tdata_corig(0)
       [lv_tdata_corig] 10.00g owi-aoC--- /dev/sde1(0)
       [lv_tmeta]       12.00m ewi-ao---- /dev/sdf1(1284)
       [lvol0_pmspare]  12.00m ewi------- /dev/sdf1(0)
       [lvol0_pmspare]  12.00m ewi------- /dev/sdf1(1287)
    
For further information on LVM cache volumes, including additional administrative examples, see the lvmcache(7) man page.
For information on creating thinly-provisioned logical volumes, see Section 4.4.5, “Creating Thinly-Provisioned Logical Volumes”.

4.4.9. Merging Snapshot Volumes

You can use the --merge option of the lvconvert command to merge a snapshot into its origin volume. If both the origin and snapshot volume are not open, the merge will start immediately. Otherwise, the merge will start the first time either the origin or snapshot are activated and both are closed. Merging a snapshot into an origin that cannot be closed, for example a root file system, is deferred until the next time the origin volume is activated. When merging starts, the resulting logical volume will have the origin’s name, minor number and UUID. While the merge is in progress, reads or writes to the origin appear as they were directed to the snapshot being merged. When the merge finishes, the merged snapshot is removed.
The following command merges snapshot volume vg00/lvol1_snap into its origin.
# lvconvert --merge vg00/lvol1_snap
You can specify multiple snapshots on the command line, or you can use LVM object tags to specify that multiple snapshots be merged to their respective origins. In the following example, logical volumes vg00/lvol1, vg00/lvol2, and vg00/lvol3 are all tagged with the tag @some_tag. The following command merges the snapshot logical volumes for all three volumes serially: vg00/lvol1, then vg00/lvol2, then vg00/lvol3. If the --background option were used, all snapshot logical volume merges would start in parallel.
# lvconvert --merge @some_tag
For information on tagging LVM objects, see Appendix D, LVM Object Tags. For further information on the lvconvert --merge command, see the lvconvert(8) man page.

4.4.10. Persistent Device Numbers

Major and minor device numbers are allocated dynamically at module load. Some applications work best if the block device is always activated with the same device (major and minor) number. You can specify these with the lvcreate and the lvchange commands by using the following arguments:
--persistent y --major major --minor minor
Use a large minor number to be sure that it has not already been allocated to another device dynamically.
If you are exporting a file system using NFS, specifying the fsid parameter in the exports file may avoid the need to set a persistent device number within LVM.

4.4.11. Changing the Parameters of a Logical Volume Group

To change the parameters of a logical volume, use the lvchange command. For a listing of the parameters you can change, see the lvchange(8) man page.
You can use the lvchange command to activate and deactivate logical volumes. To activate and deactivate all the logical volumes in a volume group at the same time, use the vgchange command, as described in Section 4.3.9, “Changing the Parameters of a Volume Group”.
The following command changes the permission on volume lvol1 in volume group vg00 to be read-only.
# lvchange -pr vg00/lvol1

4.4.12. Renaming Logical Volumes

To rename an existing logical volume, use the lvrename command.
Either of the following commands renames logical volume lvold in volume group vg02 to lvnew.
# lvrename /dev/vg02/lvold /dev/vg02/lvnew
# lvrename vg02 lvold lvnew
For more information on activating logical volumes on individual nodes in a cluster, see Section 4.7, “Activating Logical Volumes on Individual Nodes in a Cluster”.

4.4.13. Removing Logical Volumes

To remove an inactive logical volume, use the lvremove command. If the logical volume is currently mounted, unmount the volume before removing it. In addition, in a clustered environment you must deactivate a logical volume before it can be removed.
The following command removes the logical volume /dev/testvg/testlv from the volume group testvg. Note that in this case the logical volume has not been deactivated.
# lvremove /dev/testvg/testlv
Do you really want to remove active logical volume "testlv"? [y/n]: y
  Logical volume "testlv" successfully removed
You could explicitly deactivate the logical volume before removing it with the lvchange -an command, in which case you would not see the prompt verifying whether you want to remove an active logical volume.

4.4.14. Displaying Logical Volumes

There are three commands you can use to display properties of LVM logical volumes: lvs, lvdisplay, and lvscan.
The lvs command provides logical volume information in a configurable form, displaying one line per logical volume. The lvs command provides a great deal of format control, and is useful for scripting. For information on using the lvs command to customize your output, see Section 4.8, “Customized Reporting for LVM”.
The lvdisplay command displays logical volume properties (such as size, layout, and mapping) in a fixed format.
The following command shows the attributes of lvol2 in vg00. If snapshot logical volumes have been created for this original logical volume, this command shows a list of all snapshot logical volumes and their status (active or inactive) as well.
# lvdisplay -v /dev/vg00/lvol2
The lvscan command scans for all logical volumes in the system and lists them, as in the following example.
# lvscan
 ACTIVE                   '/dev/vg0/gfslv' [1.46 GB] inherit

4.4.15. Growing Logical Volumes

To increase the size of a logical volume, use the lvextend command.
When you extend the logical volume, you can indicate how much you want to extend the volume, or how large you want it to be after you extend it.
The following command extends the logical volume /dev/myvg/homevol to 12 gigabytes.
# lvextend -L12G /dev/myvg/homevol 
lvextend -- extending logical volume "/dev/myvg/homevol" to 12 GB
lvextend -- doing automatic backup of volume group "myvg"
lvextend -- logical volume "/dev/myvg/homevol" successfully extended
The following command adds another gigabyte to the logical volume /dev/myvg/homevol.
# lvextend -L+1G /dev/myvg/homevol
lvextend -- extending logical volume "/dev/myvg/homevol" to 13 GB
lvextend -- doing automatic backup of volume group "myvg"
lvextend -- logical volume "/dev/myvg/homevol" successfully extended
As with the lvcreate command, you can use the -l argument of the lvextend command to specify the number of extents by which to increase the size of the logical volume. You can also use this argument to specify a percentage of the volume group, or a percentage of the remaining free space in the volume group. The following command extends the logical volume called testlv to fill all of the unallocated space in the volume group myvg.
# lvextend -l +100%FREE /dev/myvg/testlv
  Extending logical volume testlv to 68.59 GB
  Logical volume testlv successfully resized
After you have extended the logical volume it is necessary to increase the file system size to match.
By default, most file system resizing tools will increase the size of the file system to be the size of the underlying logical volume so you do not need to worry about specifying the same size for each of the two commands.

4.4.16. Shrinking Logical Volumes

You can reduce the size of a logical volume with the lvreduce command.

Note

Shrinking is not supported on a GFS2 or XFS file system, so you cannot reduce the size of a logical volume that contains a GFS2 or XFS file system.
If the logical volume you are reducing contains a file system, to prevent data loss you must ensure that the file system is not using the space in the logical volume that is being reduced. For this reason, it is recommended that you use the --resizefs option of the lvreduce command when the logical volume contains a file system. When you use this option, the lvreduce command attempts to reduce the file system before shrinking the logical volume. If shrinking the file system fails, as can occur if the file system is full or the file system does not support shrinking, then the lvreduce command will fail and not attempt to shrink the logical volume.

Warning

In most cases, the lvreduce command warns about possible data loss and asks for a confirmation. However, you should not rely on these confirmation prompts to prevent data loss because in some cases you will not see these prompts, such as when the logical volume is inactive or the --resizefs option is not used.
Note that using the --test option of the lvreduce command does not indicate where the operation is safe, as this option does not check the file system or test the file system resize.
The following command shrinks the logical volume lvol1 in volume group vg00 to be 64 megabytes. In this example, lvol1 contains a file system, which this command resizes together with the logical volume. This example shows the output to the command.
# lvreduce --resizefs -L 64M vg00/lvol1
fsck from util-linux 2.23.2
/dev/mapper/vg00-lvol1: clean, 11/25688 files, 8896/102400 blocks
resize2fs 1.42.9 (28-Dec-2013)
Resizing the filesystem on /dev/mapper/vg00-lvol1 to 65536 (1k) blocks.
The filesystem on /dev/mapper/vg00-lvol1 is now 65536 blocks long.

  Size of logical volume vg00/lvol1 changed from 100.00 MiB (25 extents) to 64.00 MiB (16 extents).
  Logical volume vg00/lvol1 successfully resized.
Specifying the - sign before the resize value indicates that the value will be subtracted from the logical volume's actual size. The following example shows the command you would use if, instead of shrinking a logical volume to an absolute size of 64 megabytes, you wanted to shrink the volume by a value 64 megabytes.
# lvreduce --resizefs -L -64M vg00/lvol1

4.4.17. Extending a Striped Volume

In order to increase the size of a striped logical volume, there must be enough free space on the underlying physical volumes that make up the volume group to support the stripe. For example, if you have a two-way stripe that that uses up an entire volume group, adding a single physical volume to the volume group will not enable you to extend the stripe. Instead, you must add at least two physical volumes to the volume group.
For example, consider a volume group vg that consists of two underlying physical volumes, as displayed with the following vgs command.
# vgs
  VG   #PV #LV #SN Attr   VSize   VFree
  vg     2   0   0 wz--n- 271.31G 271.31G
You can create a stripe using the entire amount of space in the volume group.
# lvcreate -n stripe1 -L 271.31G -i 2 vg
  Using default stripesize 64.00 KB
  Rounding up size to full physical extent 271.31 GB
  Logical volume "stripe1" created
# lvs -a -o +devices
  LV      VG   Attr   LSize   Origin Snap%  Move Log Copy%  Devices
  stripe1 vg   -wi-a- 271.31G                               /dev/sda1(0),/dev/sdb1(0)
Note that the volume group now has no more free space.
# vgs
  VG   #PV #LV #SN Attr   VSize   VFree
  vg     2   1   0 wz--n- 271.31G    0
The following command adds another physical volume to the volume group, which then has 135 gigabytes of additional space.
# vgextend vg /dev/sdc1
  Volume group "vg" successfully extended
# vgs
  VG   #PV #LV #SN Attr   VSize   VFree
  vg     3   1   0 wz--n- 406.97G 135.66G
At this point you cannot extend the striped logical volume to the full size of the volume group, because two underlying devices are needed in order to stripe the data.
# lvextend vg/stripe1 -L 406G
  Using stripesize of last segment 64.00 KB
  Extending logical volume stripe1 to 406.00 GB
  Insufficient suitable allocatable extents for logical volume stripe1: 34480 
more required
To extend the striped logical volume, add another physical volume and then extend the logical volume. In this example, having added two physical volumes to the volume group we can extend the logical volume to the full size of the volume group.
# vgextend vg /dev/sdd1
  Volume group "vg" successfully extended
# vgs
  VG   #PV #LV #SN Attr   VSize   VFree
  vg     4   1   0 wz--n- 542.62G 271.31G
# lvextend vg/stripe1 -L 542G
  Using stripesize of last segment 64.00 KB
  Extending logical volume stripe1 to 542.00 GB
  Logical volume stripe1 successfully resized
If you do not have enough underlying physical devices to extend the striped logical volume, it is possible to extend the volume anyway if it does not matter that the extension is not striped, which may result in uneven performance. When adding space to the logical volume, the default operation is to use the same striping parameters of the last segment of the existing logical volume, but you can override those parameters. The following example extends the existing striped logical volume to use the remaining free space after the initial lvextend command fails.
# lvextend vg/stripe1 -L 406G
  Using stripesize of last segment 64.00 KB
  Extending logical volume stripe1 to 406.00 GB
  Insufficient suitable allocatable extents for logical volume stripe1: 34480 
more required
# lvextend -i1 -l+100%FREE vg/stripe1

4.4.18. Extending a RAID Volume

You can grow RAID logical volumes with the lvextend command without performing a synchronization of the new RAID regions.
If you specify the --nosync option when you create a RAID logical volume with the lvcreate command, the RAID regions are not synchronized when the logical volume is created. If you later extend a RAID logical volume that you have created with the --nosync option, the RAID extensions are not synchronized at that time, either.
You can determine whether an existing logical volume was created with the --nosync option by using the lvs command to display the volume's attributes. A logical volume will show "R" as the first character in the attribute field if it is a RAID volume that was created without an initial synchronization, and it will show "r" if it was created with initial synchronization.
The following command displays the attributes of a RAID logical volume named lv that was created without initial synchronization, showing "R" as the first character in the attribute field. The seventh character in the attribute field is "r", indicating a target type of RAID. For information on the meaning of the attribute field, see Table 4.5, “lvs Display Fields”.
# lvs vg
  LV   VG   Attr     LSize Pool Origin Snap%  Move Log Cpy%Sync  Convert
  lv   vg   Rwi-a-r- 5.00g                               100.00
If you grow this logical volume with the lvextend command, the RAID extension will not be resynchronized.
If you created a RAID logical volume without specifying the --nosync option of the lvcreate command, you can grow the logical volume without resynchronizing the mirror by specifying the --nosync option of the lvextend command.
The following example extends a RAID logical volume that was created without the --nosync option, indicated that the RAID volume was synchronized when it was created. This example, however, specifies that the volume not be synchronized when the volume is extended. Note that the volume has an attribute of "r", but after executing the lvextend command with the --nosync option the volume has an attribute of "R".
# lvs vg
  LV   VG   Attr     LSize  Pool Origin Snap%  Move Log Cpy%Sync  Convert
  lv   vg   rwi-a-r- 20.00m                               100.00        
# lvextend -L +5G vg/lv --nosync
  Extending 2 mirror images.
  Extending logical volume lv to 5.02 GiB
  Logical volume lv successfully resized
# lvs vg
  LV   VG   Attr     LSize Pool Origin Snap%  Move Log     Cpy%Sync  Convert
  lv   vg   Rwi-a-r- 5.02g                                   100.00
If a RAID volume is inactive, it will not automatically skip synchronization when you extend the volume, even if you create the volume with the --nosync option specified. Instead, you will be prompted whether to do a full resync of the extended portion of the logical volume.

Note

If a RAID volume is performing recovery, you cannot extend the logical volume if you created or extended the volume with the --nosync option specified. If you did not specify the --nosync option, however, you can extend the RAID volume while it is recovering.

4.4.19. Extending a Logical Volume with the cling Allocation Policy

When extending an LVM volume, you can use the --alloc cling option of the lvextend command to specify the cling allocation policy. This policy will choose space on the same physical volumes as the last segment of the existing logical volume. If there is insufficient space on the physical volumes and a list of tags is defined in the lvm.conf file, LVM will check whether any of the tags are attached to the physical volumes and seek to match those physical volume tags between existing extents and new extents.
For example, if you have logical volumes that are mirrored between two sites within a single volume group, you can tag the physical volumes according to where they are situated by tagging the physical volumes with @site1 and @site2 tags. You can then specify the following line in the lvm.conf file:
cling_tag_list = [ "@site1", "@site2" ]
For information on tagging physical volumes, see Appendix D, LVM Object Tags.
In the following example, the lvm.conf file has been modified to contain the following line:
cling_tag_list = [ "@A", "@B" ]
Also in this example, a volume group taft has been created that consists of the physical volumes /dev/sdb1, /dev/sdc1, /dev/sdd1, /dev/sde1, /dev/sdf1, /dev/sdg1, and /dev/sdh1. These physical volumes have been tagged with tags A, B, and C. The example does not use the C tag, but this will show that LVM uses the tags to select which physical volumes to use for the mirror legs.
# pvs -a -o +pv_tags /dev/sd[bcdefgh]
  PV         VG   Fmt  Attr PSize  PFree  PV Tags
  /dev/sdb1  taft lvm2 a--  15.00g 15.00g A
  /dev/sdc1  taft lvm2 a--  15.00g 15.00g B
  /dev/sdd1  taft lvm2 a--  15.00g 15.00g B
  /dev/sde1  taft lvm2 a--  15.00g 15.00g C
  /dev/sdf1  taft lvm2 a--  15.00g 15.00g C
  /dev/sdg1  taft lvm2 a--  15.00g 15.00g A
  /dev/sdh1  taft lvm2 a--  15.00g 15.00g A
The following command creates a 10 gigabyte mirrored volume from the volume group taft.
# lvcreate --type raid1 -m 1 -n mirror --nosync -L 10G taft
  WARNING: New raid1 won't be synchronised. Don't read what you didn't write!
  Logical volume "mirror" created
The following command shows which devices are used for the mirror legs and RAID metadata subvolumes.
# lvs -a -o +devices
  LV                VG   Attr       LSize  Log Cpy%Sync Devices
  mirror            taft Rwi-a-r--- 10.00g       100.00 mirror_rimage_0(0),mirror_rimage_1(0)
  [mirror_rimage_0] taft iwi-aor--- 10.00g              /dev/sdb1(1)
  [mirror_rimage_1] taft iwi-aor--- 10.00g              /dev/sdc1(1)
  [mirror_rmeta_0]  taft ewi-aor---  4.00m              /dev/sdb1(0)
  [mirror_rmeta_1]  taft ewi-aor---  4.00m              /dev/sdc1(0)
The following command extends the size of the mirrored volume, using the cling allocation policy to indicate that the mirror legs should be extended using physical volumes with the same tag.
# lvextend --alloc cling -L +10G taft/mirror
  Extending 2 mirror images.
  Extending logical volume mirror to 20.00 GiB
  Logical volume mirror successfully resized
The following display command shows that the mirror legs have been extended using physical volumes with the same tag as the leg. Note that the physical volumes with a tag of C were ignored.
# lvs -a -o +devices
  LV                VG   Attr       LSize  Log Cpy%Sync Devices
  mirror            taft Rwi-a-r--- 20.00g       100.00 mirror_rimage_0(0),mirror_rimage_1(0)
  [mirror_rimage_0] taft iwi-aor--- 20.00g              /dev/sdb1(1)
  [mirror_rimage_0] taft iwi-aor--- 20.00g              /dev/sdg1(0)
  [mirror_rimage_1] taft iwi-aor--- 20.00g              /dev/sdc1(1)
  [mirror_rimage_1] taft iwi-aor--- 20.00g              /dev/sdd1(0)
  [mirror_rmeta_0]  taft ewi-aor---  4.00m              /dev/sdb1(0)
  [mirror_rmeta_1]  taft ewi-aor---  4.00m              /dev/sdc1(0)

4.4.20. Controlling Logical Volume Activation

You can flag a logical volume to be skipped during normal activation commands with the -k or --setactivationskip {y|n} option of the lvcreate or lvchange command. This flag is not applied during deactivation.
You can determine whether this flag is set for a logical volume with the lvs command, which displays the k attribute as in the following example.
# lvs vg/thin1s1
LV         VG  Attr       LSize Pool  Origin
thin1s1    vg  Vwi---tz-k 1.00t pool0 thin1
By default, thin snapshot volumes are flagged for activation skip. You can activate a logical volume with the k attribute set by using the -K or --ignoreactivationskip option in addition to the standard -ay or --activate y option.
The following command activates a thin snapshot logical volume.
# lvchange -ay -K VG/SnapLV
The persistent "activation skip" flag can be turned off when the logical volume is created by specifying the -kn or --setactivationskip n option of the lvcreate command. You can turn the flag off for an existing logical volume by specifying the -kn or --setactivationskip n option of the lvchange command. You can turn the flag on again with the -ky or --setactivationskip y option.
The following command creates a snapshot logical volume without the activation skip flag
# lvcreate --type thin -n SnapLV -kn -s ThinLV --thinpool VG/ThinPoolLV
The following command removes the activation skip flag from a snapshot logical volume.
# lvchange -kn VG/SnapLV
You can control the default activation skip setting with the auto_set_activation_skip setting in the /etc/lvm/lvm.conf file.

4.4.21. Tracking and Displaying Historical Logical Volumes (Red Hat Enterprise Linux 7.3 and Later)

As of Red Hat Enterprise Linux 7.3, you can configure your system to track thin snapshot and thin logical volumes that have been removed by enabling the record_lvs_history metadata option in the lvm.conf configuration file. This allows you to display a full thin snapshot dependency chain that includes logical volumes that have been removed from the original dependency chain and have become historical logical volumes.
You can configure your system to retain historical volumes for a defined period of time by specifying the retention time, in seconds, with the lvs_history_retention_time metadata option in the lvm.conf configuration file.
A historical logical volume retains a simplified representation of the logical volume that has been removed, including the following reporting fields for the volume:
  • lv_time_removed: the removal time of the logical volume
  • lv_time: the creation time of the logical volume
  • lv_name: the name of the logical volume
  • lv_uuid: the UUID of the logical volume
  • vg_name: the volume group that contains the logical volume.
When a volume is removed, the historical logical volume name acquires a hypen as a prefix. For example, when you remove the logical volume lvol1, the name of the historical volume is -lvol1. A historical logical volume cannot be reactivated.
Even when the record_lvs_history metadata option enabled, you can prevent the retention of historical logical volumes on an individual basis when you remove a logical volume by specifying the --nohistory option of the lvremove command.
To include historical logical volumes in volume display, you specify the -H|--history option of an LVM display command. You can display a full thin snapshot dependency chain that includes historical volumes by specifying the lv_full_ancestors and lv_full_descendants reporting fields along with the -H option.
The following series of commands provides examples of how you can display and manage historical logical volumes.
  1. Ensure that historical logical volumes are retained by setting record_lvs_history=1 in the lvm.conf file. This metadata option is not enabled by default.
  2. Enter the following command to display a thin provisioned snapshot chain.
    In this example:
    • lvol1 is an origin volume, the first volume in the chain.
    • lvol2 is a snapshot of lvol1.
    • lvol3 is a snapshot of lvol2.
    • lvol4 is a snapshot of lvol3.
    • lvol5 is also a snapshot of lvol3.
    Note that even though the example lvs display command includes the -H option, no thin snapshot volume has yet been removed and there are no historical logical volumes to display.
    # lvs -H -o name,full_ancestors,full_descendants
      LV    FAncestors        FDescendants
      lvol1                   lvol2,lvol3,lvol4,lvol5
      lvol2 lvol1             lvol3,lvol4,lvol5
      lvol3 lvol2,lvol1       lvol4,lvol5
      lvol4 lvol3,lvol2,lvol1
      lvol5 lvol3,lvol2,lvol1
      pool
    
  3. Remove logical volume lvol3 from the snapshot chain, then run the following lvs command again to see how historical logical volumes are displayed, along with their ancestors and descendants.
    # lvremove -f vg/lvol3
      Logical volume "lvol3" successfully removed
    # lvs -H -o name,full_ancestors,full_descendants
      LV     FAncestors         FDescendants
      lvol1                     lvol2,-lvol3,lvol4,lvol5
      lvol2  lvol1              -lvol3,lvol4,lvol5
      -lvol3 lvol2,lvol1        lvol4,lvol5
      lvol4  -lvol3,lvol2,lvol1
      lvol5  -lvol3,lvol2,lvol1
      pool
    
  4. You can use the lv_time_removed reporting field to display the time a historical volume was removed.
    # lvs -H -o name,full_ancestors,full_descendants,time_removed
      LV     FAncestors         FDescendants              RTime
      lvol1                     lvol2,-lvol3,lvol4,lvol5  
      lvol2  lvol1              -lvol3,lvol4,lvol5
      -lvol3 lvol2,lvol1        lvol4,lvol5               2016-03-14 14:14:32 +0100
      lvol4  -lvol3,lvol2,lvol1
      lvol5  -lvol3,lvol2,lvol1
      pool
    
  5. You can reference historical logical volumes individually in a display command by specifying the vgname/lvname format, as in the following example. Note that the fifth bit in the lv_attr field is set to h to indicate the volume is a historical volume.
    # lvs -H vg/-lvol3
      LV     VG   Attr       LSize 
      -lvol3 vg   ----h-----    0
    
  6. LVM does not keep historical logical volumes if the volume has no live descendant. This means that if you remove a logical volume at the end of a snapshot chain, the logical volume is not retained as a historical logical volume.
    # lvremove -f vg/lvol5
      Automatically removing historical logical volume vg/-lvol5.
      Logical volume "lvol5" successfully removed
    # lvs -H -o name,full_ancestors,full_descendants
      LV     FAncestors         FDescendants
      lvol1                     lvol2,-lvol3,lvol4
      lvol2  lvol1              -lvol3,lvol4
      -lvol3 lvol2,lvol1        lvol4
      lvol4  -lvol3,lvol2,lvol1
      pool
    
  7. Run the following commands to remove the volume lvol1 and lvol2 and to see how the lvs command displays the volumes once they have been removed.
    # lvremove -f vg/lvol1 vg/lvol2
      Logical volume "lvol1" successfully removed
      Logical volume "lvol2" successfully removed
    # lvs -H -o name,full_ancestors,full_descendants
      LV     FAncestors           FDescendants
      -lvol1                      -lvol2,-lvol3,lvol4
      -lvol2 -lvol1               -lvol3,lvol4
      -lvol3 -lvol2,-lvol1        lvol4
      lvol4  -lvol3,-lvol2,-lvol1
      pool
    
  8. To remove a historical logical volume completely, you can run the lvremove command again, specifying the name of the historical volume that now includes the hyphen, as in the following example.
    # lvremove -f vg/-lvol3
    Historical logical volume "lvol3" successfully removed
    # lvs -H -o name,full_ancestors,full_descendants
      LV     FAncestors    FDescendants
      -lvol1               -lvol2,lvol4
      -lvol2 -lvol1        lvol4
      lvol4  -lvol2,-lvol1
      pool
    
  9. A historical logical volumes is retained as long as there is a chain that includes live volumes in its descendants. This means that removing a historical logical volume also removes all of the logical volumes in the chain if no existing descendant is linked to them, as shown in the following example.
    # lvremove -f vg/lvol4
      Automatically removing historical logical volume vg/-lvol1.
      Automatically removing historical logical volume vg/-lvol2.
      Automatically removing historical logical volume vg/-lvol4.
      Logical volume "lvol4" successfully removed