Chapter 9. Configuring 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.

To create a RAID logical volume, you specify a raid type as the --type argument of the lvcreate command. Table 9.1, “RAID Segment Types” describes the possible RAID segment types. 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.

Table 9.1. 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

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 most users, specifying one of the five available primary types (raid1, raid4, raid5, raid6, raid10) should be sufficient. For more information on the different algorithms used by RAID 5/6, see chapter four of the Common RAID Disk Data Format Specification at http://www.snia.org/sites/default/files/SNIA_DDF_Technical_Position_v2.0.pdf.

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).

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/.

9.1. Creating RAID logical volumes

This section provides example commands that create different types of RAID logical volume.

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 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

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

9.2. Creating a RAID0 (striped) logical volume

A RAID0 logical volume spreads logical volume data across multiple data subvolumes in units of stripe size.

The format for the command to create a RAID0 volume is as follows. 

lvcreate --type raid0[_meta] --stripes Stripes --stripesize StripeSize VolumeGroup [PhysicalVolumePath ...]

Table 9.2. 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.

This example procedure creates an LVM RAID0 logical volume called mylv that stripes data across the disks at /dev/sda1, /dev/sdb1, and /dev/sdc1.

  1. Label the disks you will use in the volume group as LVM physical volumes with the pvcreate command.

    Warning

    This command destroys any data on /dev/sda1, /dev/sdb1, and /dev/sdc1. 

    # pvcreate /dev/sda1 /dev/sdb1 /dev/sdc1
  Physical volume "/dev/sda1" successfully created
  Physical volume "/dev/sdb1" successfully created
  Physical volume "/dev/sdc1" successfully created

  2. Create the volume group myvg. The following command creates the volume group myvg. 

    # vgcreate myvg /dev/sda1 /dev/sdb1 /dev/sdc1
  Volume group "myvg" successfully created

    You can use the vgs command to display the attributes of the new volume group. 

    # vgs
  VG   #PV #LV #SN Attr   VSize  VFree
  myvg   3   0   0 wz--n- 51.45G 51.45G

  3. Create a RAID0 logical volume from the volume group you have created. The following command creates the RAID0 volume mylv from the volume group myvg. This example creates a logical volume that is 2 gigabytes in size, with three stripes and a stripe size of 4 kilobytes. 

    # lvcreate --type raid0 -L 2G --stripes 3 --stripesize 4 -n mylv myvg
  Rounding size 2.00 GiB (512 extents) up to stripe boundary size 2.00 GiB(513 extents).
  Logical volume "mylv" created.

  4. Create a file system on the RAID0 logical volume. The following command creates an ext4 file system on the logical volume. 

    # mkfs.ext4 /dev/myvg/mylv
mke2fs 1.44.3 (10-July-2018)
Creating filesystem with 525312 4k blocks and 131376 inodes
Filesystem UUID: 9d4c0704-6028-450a-8b0a-8875358c0511
Superblock backups stored on blocks:
        32768, 98304, 163840, 229376, 294912

Allocating group tables: done
Writing inode tables: done
Creating journal (16384 blocks): done
Writing superblocks and filesystem accounting information: done

    The following commands mount the logical volume and report the file system disk space usage. 

    # mount /dev/myvg/mylv /mnt
# df
Filesystem             1K-blocks     Used  Available Use% Mounted on
/dev/mapper/myvg-mylv    2002684     6168    1875072   1% /mnt

9.3. Controlling the rate at which RAID volumes are initialized

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.

9.4. 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.

9.5. 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)

9.6. 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)

9.7. 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.
  • 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.
  • 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.

9.8. 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.

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)

9.9. 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 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, 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)

9.10. 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 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/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)

9.11. 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.

9.11.1. 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 initiating recovery of the failed device with the --refresh option of the lvchange command. Alternately, you can replace the failed device.

9.11.2. 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.

9.12. Replacing a RAID device in a logical volume

You can replace a RAID device in a logical volume with the lvconvert command.

  • If there has been no failure on a RAID device, use the --replace argument of the lvconvert command to replace the device.
  • If the RAID device has failed, use the --repair argument of the lvconvert command to replace the failed device.

9.12.1. Replacing a RAID device that has not failed

To replace a RAID device in a logical volume, use the --replace argument of the lvconvert command. Note that this command will not work if the RAID device has failed.

The format for the lvconvert --replace command 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)

9.12.2. Replacing a failed RAID device in a logical volume

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 failed device in a RAID volume in a one-step solution by using the --repair argument of the lvconvert command.

In the following example, a RAID logical volume is laid out as follows. 

# lvs -a -o name,copy_percent,devices my_vg
  LV               Cpy%Sync 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/sdc1(1)
  [my_lv_rimage_2]          /dev/sdd1(1)
  [my_lv_rmeta_0]           /dev/sde1(0)
  [my_lv_rmeta_1]           /dev/sdc1(0)
  [my_lv_rmeta_2]           /dev/sdd1(0)

If the /dev/sdc device fails, the output of the lvs command is as follows. 

# lvs -a -o name,copy_percent,devices my_vg
  /dev/sdc: open failed: No such device or address
  Couldn't find device with uuid A4kRl2-vIzA-uyCb-cci7-bOod-H5tX-IzH4Ee.
  WARNING: Couldn't find all devices for LV my_vg/my_lv_rimage_1 while checking used and assumed devices.
  WARNING: Couldn't find all devices for LV my_vg/my_lv_rmeta_1 while checking used and assumed devices.
  LV               Cpy%Sync 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]          [unknown](1)
  [my_lv_rimage_2]          /dev/sdd1(1)
  [my_lv_rmeta_0]           /dev/sde1(0)
  [my_lv_rmeta_1]           [unknown](0)
  [my_lv_rmeta_2]           /dev/sdd1(0)

Use the following commands to replace the failed device and display the logical volume. 

# lvconvert --repair my_vg/my_lv
  /dev/sdc: open failed: No such device or address
  Couldn't find device with uuid A4kRl2-vIzA-uyCb-cci7-bOod-H5tX-IzH4Ee.
  WARNING: Couldn't find all devices for LV my_vg/my_lv_rimage_1 while checking used and assumed devices.
  WARNING: Couldn't find all devices for LV my_vg/my_lv_rmeta_1 while checking used and assumed devices.
Attempt to replace failed RAID images (requires full device resync)? [y/n]: y
  Faulty devices in my_vg/my_lv successfully replaced.
# lvs -a -o name,copy_percent,devices my_vg
  /dev/sdc: open failed: No such device or address
  /dev/sdc1: open failed: No such device or address
  Couldn't find device with uuid A4kRl2-vIzA-uyCb-cci7-bOod-H5tX-IzH4Ee.
  LV               Cpy%Sync Devices
  my_lv            43.79    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/sdb1(1)
  [my_lv_rimage_2]          /dev/sdd1(1)
  [my_lv_rmeta_0]           /dev/sde1(0)
  [my_lv_rmeta_1]           /dev/sdb1(0)
  [my_lv_rmeta_2]           /dev/sdd1(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.

The following command refreshes a logical volume. 

# lvchange --refresh my_vg/my_lv

9.13. Checking data coherency in a RAID logical volume (RAID scrubbing)

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 RAID scrubbing operation, the lvs command 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.

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.

9.14. Converting a RAID level (RAID takeover)

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.

9.15. Changing attributes of a RAID volume (RAID reshape)

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.

9.16. 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.

9.17. Changing the region size on a RAID logical volume

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.