A Linux User Reference

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Logical Volumes

  • Logical Volume Manager (LVM)
    • A LVM provides greater flexibility in administrating storage devices.
    • Storage volumes created under the control of the logical volume manager can be resized and moved around almost at will.
    • LVM configuration file is /etc/lvm/lvm.conf which contains configuration information on how commands will behave.
  • Place physical volumes under LVM management
    • Place disks or partitions (physical volumes) under LVM management.
    • If the disk already has a partition table on it, not even --force option will make 'pvcreate' run.
    • Will also complain if partition type is not 0x8e, change if required.
    pvcreate [options] physicalvolume [physicalvolume ...]

    Destroy partition table on a disk and initialise it for LVM

    # dd if=/dev/zero of=/dev/sda bs=1k count =1
    # blockdev --rereadpt /dev/sda
    # pvcreate  /dev/sda7
  • Create a new volume group
    • uses the block special device(s) which have been previously configured for LVM with pvcreate
    • a volume group consists of one or more physical disks or partitions - physical volumes.
    vgcreate [options] VGname device1 [device? ...]

    Create volume on /dev/sda7

    # vgcreate FedoraVG /dev/sda7
  • Create a new logical volume
    • allocates logical extents from the free physical extent pool of a volume group.
    • the resulting LV can then be used to contain a filesystem.
    • under LVM more than one filesystem can reside on a single physical partition.
    lvcreate [options] VGname
    Common options:
     -L size                 Size can be postfixed with K, M, G, T(terabytes), default is M.
     -l size                 In logical extents (# vgdisplay shows number of extents in VG)
                             can be more accurate to work with than M, K, ...
     -n lvname               LV name, defaults to "lvol#" where # is the LVM internal.

    Create a 1.41GB logical volume named optLV in FedoraVG volume group

    # lvcreate -L 1.41G -n optLV FedoraVG
  • Display attributes of a physical volume
    pvdisplay [options] device [device ...]
     -c | --colon             Colon separated output for easier parsing in scripts or programs
     -s | --short             Only display the size of the given physical volumes
     -m | --maps              Display the mapping of physical extents to logical volumes and
                              logical extents

    Display attributes of /dev/sda7

    # pvdisplay -c /dev/sda7
    # pvdisplay /dev/sda7
     --- Physical volume ---
      PV Name               /dev/sda7
      VG Name               FedoraVG
      PV Size               24.51 GB / not usable 5.69 MB
      Allocatable           yes
      PE Size (KByte)       32768
      Total PE              784
      Free PE               19
      Allocated PE          765
      PV UUID               gaW3J4-q8tI-GHGc-GubJ-dl3g-mjL8-eEWwhY
  • Report information about physical volumes

    Provides greater control over the format of the output than 'pvdisplay'.

    pvs [options] [device ...]
    Some options:
     --aligned               Use with --separator to align the output columns.
     --noheadings            Suppress the headings line - normally the first line of output,
                             useful if 'grepping'
     --nosuffix              Suppress the suffix on output sizes.  Use with --units (except
                             h and H) if processing the output.

    Display attributes of a physical volume

    # pvs
      PV         VG       Fmt  Attr PSize  PFree
      /dev/sda7  FedoraVG lvm2 a-   24.50G 608.00M
  • Display attributes of volume groups
    vgdisplay [options] [VGname ...]
    Some options:
     -A | --activevolumegroups            Only select the active volume groups.
     -c | --colon                         Generate colon separated output.
     -s | --short                         Short listing showing the existence of volume groups.
     -v | --verbose                       Display verbose information.

    Display attributes of a Volume Group

    # vgdisplay FedoraVG
     --- Volume group ---
      VG Name               FedoraVG
      System ID
      Format                lvm2
      VG Size               24.50 GB
      PE Size               32.00 MB
      Total PE              784
      Alloc PE / Size       765 / 23.91 GB
      Free  PE / Size       19 / 608.00 MB
      VG UUID               7Phmek-xgrT-SesT-KVQo-4AZ8-z8Yr-0lhB9t
    # vgdisplay -v FedoraVG | grep "LV Name"
        Using volume group(s) on command line
        Finding volume group "FedoraVG"
      LV Name                /dev/FedoraVG/optLV
      LV Name                /dev/FedoraVG/tmpLV
      LV Name                /dev/FedoraVG/homeLV
      LV Name                /dev/FedoraVG/usrLV
      LV Name                /dev/FedoraVG/varLV
      LV Name                /dev/FedoraVG/shareLV
    # vgdisplay -s FedoraVG
      "FedoraVG" 24.50 GB  [23.91 GB  used / 608.00 MB free]
  • Report information about volume groups

    Provides reater control over the format of the output than 'vgdisplay'.

    vgs [options] [device ...]
    Some options:
     --aligned               Use with --separator to align the output columns.
     --noheadings            Suppress the headings line.
     --nosuffix              Suppress the suffix on output sizes.

    Display attributes of a Volume Group

    # vgs
      VG       #PV #LV #SN Attr   VSize  VFree
      FedoraVG   1   6   0 wz--n- 24.50G 608.00M
  • Display attributes of a logical volume
    lvdisplay [options] LVname [LVname ...]
    Some options:
     -c | --colon            Colon separated output for easier parsing in scripts or programs.
     -m | --maps             Display the mapping of physical extents to logical volumes and
                             logical extents.

    Display attributes of a Logical Volume

    # lvdisplay /dev/FedoraVG/optLV
     --- Logical volume ---
      LV Name                /dev/FedoraVG/optLV
      VG Name                FedoraVG
      LV UUID                qVldo5-Y9Zm-QlNj-CMFo-JN8J-A7Ii-Q4143Y
      LV Write Access        read/write
      LV Status              available
      # open                 1
      LV Size                1.41 GB
      Current LE             45
      Segments               1
      Allocation             inherit
      Read ahead sectors     0
      Block device           253:0
  • Report information about logical volumes

    Provides greater control over the format of the output than 'lvdisplay'.

    lvs [options] [device ...]
    Some options:
     --aligned               Use with --separator to align the output columns.
     --noheadings            Suppress the headings line.
     --nosuffix              Suppress the suffix on output sizes..

    Display information about a Logical Volume

    # lvs
      LV      VG       Attr   LSize  Origin Snap%  Move Log Copy%
      homeLV  FedoraVG -wi-ao  8.00G
      optLV   FedoraVG -wi-ao  1.41G
      shareLV FedoraVG -wi-ao  1.50G
      tmpLV   FedoraVG -wi-ao  1.00G
      usrLV   FedoraVG -wi-ao 10.00G
      varLV   FedoraVG -wi-ao  2.00G
  • Scan all disks for physical volumes
    pvscan [options]
    Some options:
     -n | --novolumegroup          Only show physical volumes not belonging to any volume group
     -e | --exported               Only show physical volumes belonging to exported volume groups
     -s | --short                  Short listing format
     -u | --uuid                   Show UUIDs (Uniform Unique Identifiers) in addition to device
                                   special names

    Scan all disks for PVs

    # pvscan
      PV /dev/sda7   VG FedoraVG   lvm2 [24.50 GB / 608.00 MB free]
      Total: 1 [24.50 GB] / in use: 1 [24.50 GB] / in no VG: 0 [0   ]
    # pvscan -n
      WARNING: only considering physical volumes in no volume group
      No matching physical volumes found
  • Moves physical extents

    If no destination PV or LV then the moved physical extents are distributed using the normal allocation rules of the volume group.

    pvmove [options] 
    Some options:
     -n | --name  LogicalVolume        Move only the extents belonging to LogicalVolume from
                                       SourcePhysicalVolume instead of all allocated extents to
                                       the destination physical volume(s)
     -t | --test
     -v                                Verbose
      [:PE[-PE]]                       Physical extent range

    The following examples are from here

    Move all logical extents of any logical volumes on /dev/hda4 to free physical extents elsewhere in the volume group

    pvmove -v /dev/hda4

    Move three logical extents (3, 5 and 11) of logical volume my_lv to free ones on /dev/hdb3

    pvmove -n my_lv:3:5:11 /dev/sdb6 /dev/hdb3 
  • Add physical volumes to a volume group
    vgextend [options] 
             VGname physicaldevice [physicaldevice ...]
    Some options:
     -a | --autobackup <y|n>
     -t | --test

    Add /dev/sda5 and /dev/sdc1 to volume group foo_vg

    vgextend foo_vg /dev/sda5 /dev/sdc1
  • Remove a physical volume(s) from a volume group
    vgreduce VGname physicalvolume [physicalvolume ...]
    Some options:
     -a | --all               Removes all empty physical volumes if none are given on command line
     -t | --test
     --removemissing          Removes all missing physical volumes from the volume group and makes
                              the volume group consistent again. Run this option with '--test' first
                              to find out what it would remove before running it for real.

    Remove /dev/sda5 and /dev/sdc1 to volume group foo_vg

    vgremove foo_vg /dev/sda5 /dev/sdc1
  • Reduce the size of a logical volume
    lvreduce [options] LogicalVolume[Path]
     -A | --autobackup             y/n
     -d | --debug
     -f | --force
     -h | -? | --help
     -l | --extents [-]LogicalExtentsNumber[%{VG|LV|FREE}]
     -L | --size [-]LogicalVolumeSize[kKmMgGtT]
     -t | --test
     -v | --verbose

    Reduce size of lv01 in vg foo_vg by 3 extents

    lvreduce -l -3 /dev/foo_vg/lv01

    Safe way to shrink a Logicial volume

    • shrink the FileSystem a bit more than required
    • then shrink the LV
    • then resize the FileSystem to fit LV completely
    # resize2fs /dev/FedoraVG/optLV 1.35G           (shrink filesystem to 1.35GB)
    # lvreduce -L 1.41G /dev/FedoraVG/optLV         (reduce the logical volume)
    # resize2fs /dev/FedoraVG/optLV                 (resize the filesystem to fit)
  • Extend the size of a logical volume
    lvextend [options] LVpath [physicalvolume ...]
    Some options:
     -l | --extents [+]LogicalExtentsNumber[%{VG|LV|FREE}]
                    Extend or set the logical volume size in units of logical extents
                    + => add to the actual size of the logical volume, without + => absolute value
                    %VG => a percentage of the total space in the Volume Group
                    %LV => relative to the existing size of the Logical Volume
                    %FREE => percentage of the remaining free space in the Volume Group
     -L | --size [+]LogicalVolumeSize[kKmMgGtTpPeE]
                    Extend or set the logical volume size in units of megabytes
                    M=megabytes, G=gigabytes, T=terabytes, P=petabytes, E=exabytes
                    + sign the value is added to the actual size of the logical volume
                    without a + the value is taken as an absolute one
     -t | --test

    Increase size of a filesystem by 200MB

    # lvextend -L +200M /dev/FedoraVG/optLV            (increase LV first)
    # resize2fs /dev/FedoraVG/optLV                    (resize the fs to fill the LV)
  • Remove a volume group
    • The volume group(s) must not have any logical volumes allocated
    • the volume group needs to be made unavailable (varyoff) first
    vgremove [options] VolumeGroupName [...]
     -d | --debug
     -h ! -? | --help
     -t | --test
     -v | --verbose

    Remove volume group foo_vg

    vgchange -a n foo_vg            (varyoff the VG first)
    vgremove foo_vg
  • Remove a logical volume

    All data will be lost.

    lvremove [options] LVpath [LVpath ...]
    Some options:
     -A | --autobackup y|n
     -f | --force                   Remove active lvs without confirmation
     -t | --test                    Report what would happen, do not do the removal

    Test removal of logical volume optLV

    lvremove --test /dev/FedoraVG/optLV
  • Change a Volume Group's attributes
    vgchange [options] VGname
    Some options:
     -a [n|y]                             Deactivate, activate a VG (varyon, varyoff)
     -l | --logicalvolume int             Change the maximum logical volume number of an
                                          existing inactive volume group.
     -p | --maxphysicalvolumes int        Change the maximum number of physical volumes that
                                          can belong to this volume group.

    Varyoff then varyon volume group FedoraVG

    vgchange -a n FedoraVG 
    vgchange -a y FedoraVG 
  • Scan all disks for volume groups and rebuild caches

    Can define a filter in lvm.conf.

    vgscan [options]

    Scan all VGs

    # vgscan -v
    Wiping cache of LVM-capable devices
    Wiping internal VG cache
    Reading all physical volumes.  This may take a while...
    Finding all volume groups
    Finding volume group "FedoraVG"
    Found volume group "FedoraVG" using metadata type lvm2
  • Logical volume snapshot

    A LV can be frozen while still being used, a backup can be made of the frozen copy.

    Create a snapshot

    # lvcreate -L 100M -s -n lv_backup /dev/FedoraVG/optLV

    Creates a snapshot of optLV in a 100M logical volume named lv_backup.

    Remove a snapshot

    # lvremove /dev/VGname/lv_backup

    Usually after it has been backed up.

  • Convert a logical volume

    Converts from linear to mirror or snapshot. Also used to add and remove disk logs from mirror devices.

    lvconvert -m LV[Path] [PV[Path] ...]
     -m | --mirrors Mirrors                  Number of mirror copies (-m 1 creates a single mirror)
     --corelog                               Have mirror copy use a memory-based as opposed to disk
                                             (persistent) based log.
     -R | --regionsize MirrorLogRegionSize   A mirror is divided into regions of this size(MB).
                                             The mirror log uses this granularity to track which 
                                             regions are in sync.
     -A | --alloc AllocationPolicy
    lvconvert -s | --snapshot OriginalLV[Path] SnapshotLV[Path]
     -s | --snapshot                         Create a snapshot from existing logical volume using
                                             another existing logical volume as its origin.
     -c | --chunksize ChunkSize              Power of 2 chunk size for the snapshot logical volume
                                             between 4k and 512k
     -Z | --zero y/n                         Controls zeroing of the first KB of data in the snapshot.
                                             If volume is read-only the snapshot will not be zeroed.

    Man page examples:

    Convert the linear logical volume "vg00/lvol1" to a mirror logical volume

    # lvconvert -m1 vg00/lvol1

    Convert a two-way mirror with a disk log to a two-way mirror with an in-memory log

    # lvconvert -m1 --corelog vg00/lvol1

    Convert a mirror logical volume to a linear logical volume

    # lvconvert -m0 vg00/lvol1

    Convert logical volume "vg00/lvol2" to snapshot of original volume "vg00/lvol1"

    # lvconvert -s vg00/lvol1 vg00/lvol2