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Chapter 4. Managing LVM volume groups

A volume group (VG) is a collection of physical volumes (PVs), which creates a pool of disk space out of which logical volumes (LVs) can be allocated.

Within a volume group, the disk space available for allocation is divided into units of a fixed-size called extents. An extent is the smallest unit of space that can be allocated. Within a physical volume, extents are referred to as physical extents.

A logical volume is allocated into logical extents of the same size as the physical extents. The extent size is thus the same for all logical volumes in the volume group. The volume group maps the logical extents to physical extents.

4.1. Creating LVM volume group

This procedure describes how to create an LVM volume group (VG) myvg, by using the /dev/vdb1 and /dev/vdb2 physical volumes.

Prerequisites

  • The lvm2 package is installed.
  • One or more physical volumes are created. For more information on creating physical volumes, see Creating LVM physical volume.

Procedure

  1. Create a volume group: 

    # vgcreate myvg /dev/vdb1 /dev/vdb2
 Volume group "myvg" successfully created.

    This creates a VG with the name of myvg. The PVs /dev/vdb1 and /dev/vdb2 are the base storage level for the myvg VG .

  2. View the created volume groups by using any one of the following commands as per your requirement:

    1. The vgs command provides volume group information in a configurable form, displaying one line per volume groups: 

      # vgs
  VG   #PV #LV #SN Attr   VSize   VFree
  myvg   4   1   0 wz--n-   3.98g 1008.00m

    2. The vgdisplay command displays volume group properties such as size, extents, number of physical volumes, and other options in a fixed form. The following example shows the output of the vgdisplay command for the volume group myvg. If you do not specify a volume group, all existing volume groups are displayed: 

      # vgdisplay myvg _ --- Volume group --- VG Name _myvg
  System ID
  Format                lvm2
  Metadata Areas        4
  Metadata Sequence No  6
  VG Access             read/write
[..]

    3. The vgscan command scans all supported LVM block devices in the system for volume group: 

      # vgscan
  Found volume group "myvg" using metadata type lvm2

  3. Optional: Increase a volume group’s capacity by adding one or more free physical volumes: 

    # vgextend myvg /dev/vdb3
Physical volume "/dev/vdb3" successfully created.
Volume group "myvg" successfully extended

Additional resources

  • pvcreate(8), vgextend(8), vgdisplay(8), vgs(8), vgscan(8), and lvm(8) man pages

4.2. Combining LVM volume groups

To combine two volume groups into a single volume group, use the vgmerge command. You can merge an inactive "source" volume with an active or an inactive "destination" volume if the physical extent sizes of the volume are equal and the physical and logical volume summaries of both volume groups fit into the destination volume groups limits.

Procedure

  • Merge the inactive volume group databases into the active or inactive volume group myvg giving verbose runtime information: 

    # vgmerge -v myvg databases

Additional resources

  • vgmerge(8) man page

4.3. Removing physical volumes from a volume group

To remove unused physical volumes from a volume group, use the vgreduce command. The vgreduce command shrinks a volume group’s capacity by removing one or more empty physical volumes. This frees those physical volumes to be used in different volume groups or to be removed from the system.

Procedure

  1. If the physical volume is still being used, migrate the data to another physical volume from the same volume group : 

    # pvmove /dev/vdb3
  /dev/vdb3: Moved: 2.0%
 ...
  /dev/vdb3: Moved: 79.2%
 ...
  /dev/vdb3: Moved: 100.0%

  2. If there are no enough free extents on the other physical volumes in the existing volume group:

    1. Create a new physical volume from /dev/vdb4: 

      # pvcreate /dev/vdb4
  Physical volume "/dev/vdb4" successfully created

    2. Add the newly created physical volume to the myvg volume group: 

      # vgextend myvg /dev/vdb4
  Volume group "myvg" successfully extended

    3. Move the data from /dev/vdb3 to /dev/vdb4: 

      # pvmove /dev/vdb3 /dev/vdb4
  /dev/vdb3: Moved: 33.33%
  /dev/vdb3: Moved: 100.00%

  3. Remove the physical volume /dev/vdb3 from the volume group: 

    # vgreduce myvg /dev/vdb3
Removed "/dev/vdb3" from volume group "myvg"

Verification

  • Verify if the /dev/vdb3 physical volume is removed from the myvg volume group: 

    # pvs
  PV           VG    Fmt   Attr   PSize        PFree      Used
  /dev/vdb1 myvg  lvm2   a--    1020.00m    0          1020.00m
  /dev/vdb2 myvg  lvm2   a--    1020.00m    0          1020.00m
  /dev/vdb3   	    lvm2   a--    1020.00m   1008.00m    12.00m

Additional resources

  • vgreduce(8), pvmove(8), and pvs(8) man pages

4.4. Splitting a LVM volume group

This procedure describes how to split the existing volume group. If there is enough unused space on the physical volumes, a new volume group can be created without adding new disks.

In the initial setup, the volume group myvg consists of /dev/vdb1, /dev/vdb2, and /dev/vdb3. After completing this procedure, the volume group myvg will consist of /dev/vdb1 and /dev/vdb2, and the second volume group, yourvg, will consist of /dev/vdb3.

Prerequisites

  • You have sufficient space in the volume group. Use the vgscan command to determine how much free space is currently available in the volume group.
  • Depending on the free capacity in the existing physical volume, move all the used physical extents to other physical volume using the pvmove command. For more information, see Removing physical volumes from a volume group.

Procedure

  1. Split the existing volume group myvg to the new volume group yourvg: 

    # vgsplit myvg yourvg /dev/vdb3
  Volume group "yourvg" successfully split from "myvg"
    Note

    If you have created a logical volume using the existing volume group, use the following command to deactivate the logical volume: 

    # lvchange -a n /dev/myvg/mylv

    For more information on creating logical volumes, see Managing LVM logical volumes.

  2. View the attributes of the two volume group: 

    # vgs
  VG     #PV #LV #SN Attr   VSize  VFree
  myvg     2   1   0 wz--n- 34.30G 10.80G
  yourvg   1   0   0 wz--n- 17.15G 17.15G

Verification

  • Verify if the newly created volume group yourvg consists of /dev/vdb3 physical volume: 

    # pvs
  PV           VG      Fmt   Attr   PSize        PFree      Used
  /dev/vdb1 myvg   lvm2   a--    1020.00m    0          1020.00m
  /dev/vdb2 myvg   lvm2   a--    1020.00m    0          1020.00m
  /dev/vdb3 yourvg lvm2   a--    1020.00m   1008.00m    12.00m

Additional resources

  • vgsplit(8), vgs(8), and pvs(8) man pages

4.5. Renaming LVM volume groups

This procedure renames an existing volume group myvg to myvg1.

Procedure

  1. Deactivate the volume group. If it is a clustered volume group, deactivate the volume group on all nodes where it is active by using the following command on each such node: 

    # vgchange --activate n myvg

  2. Rename an existing volume group: 

    # vgrename myvg myvg1
Volume group "myvg" successfully renamed to "myvg1"

    You can also rename the volume group by specifying the full paths to the devices: 

    # vgrename /dev/myvg /dev/myvg1

Additional resources

  • vgrename(8) man page

4.6. Moving a volume group to another system

You can move an entire LVM volume group to another system. It is recommended that you use the vgexport and vgimport commands when you do this.

Note

You can use the --force argument of the vgimport command. This allows you to import volume groups that are missing physical volumes and subsequently run the vgreduce --removemissing command.

The vgexport command makes an inactive volume group inaccessible to the system, which allows you to detach its physical volumes. The vgimport command makes a volume group accessible to a machine again after the vgexport command has made it inactive.

To move a volume group from one system to another, perform the following steps:

  1. Make sure that no users are accessing files on the active volumes in the volume group, then unmount the logical volumes.
  2. Use the -a n argument of the vgchange command to mark the volume group as inactive, which prevents any further activity on the volume group.

  3. Use the vgexport command to export the volume group. This prevents it from being accessed by the system from which you are removing it.

    After you export the volume group, the physical volume will show up as being in an exported volume group when you execute the pvscan command, as in the following example. 

    # pvscan
  PV /dev/sda1    is in exported VG myvg [17.15 GB / 7.15 GB free]
  PV /dev/sdc1    is in exported VG myvg [17.15 GB / 15.15 GB free]
  PV /dev/sdd1   is in exported VG myvg [17.15 GB / 15.15 GB free]
  ...

    When the system is next shut down, you can unplug the disks that constitute the volume group and connect them to the new system.

  4. When the disks are plugged into the new system, use the vgimport command to import the volume group, making it accessible to the new system.
  5. Activate the volume group with the -a y argument of the vgchange command.
  6. Mount the file system to make it available for use.

4.7. Removing LVM volume groups

This procedure describes how to remove an existing volume group.

Prerequisites

Procedure

  1. If the volume group exists in a clustered environment, stop the lockspace of the volume group on all other nodes. Use the following command on all nodes except the node where you are performing the removing: 

    # vgchange --lockstop vg-name

    Wait for the lock to stop.

  2. Remove the volume group: 

    # vgremove vg-name
  Volume group "vg-name" successfully removed

Additional resources

  • vgremove(8) man page