RHEL: Logical Volume Management

Posted on May 20, 2009

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Logical Volume Management (LVM)

Logical Volume Management is a method of managing disk drives and similar mass-storage devices. In Linux, a logical volume manager perform this task for the Linux kernel. The manager is capable to allocate space on the mass-storage devices, dynamically move or resize the space while it is being used. The manager controls physical volumes, physical extends, logical extends, volume groups and logical volumes in order to perform this flexible space allocation.

A physical volume (PV) may be a hard disk partition, a RAID device or a SAN. A PV is split into chunks called physical extends (PEs). PEs are mapped one-to-one to logical extents (LEs) or are mirrored onto one LE. A volume group (VG) is a pool of LEs. LEs in a volume group can be concatenated into Logical Volumes (LVs). And finally these LVs are useable as a raw block devices just as disk partitions are; file systems can be created on an LV.

A VG can grow its storage pool by adding in new PVs or shrink by reducing PVs. An LV can grow by adding in more LEs or shrink by returning LEs to the pool.

Linux LVM

In Linux operating systems, a logical volume manager can:

  • Resize VGs online by adding in or reducing PVs.
  • Resize LVs online by adding in or reducing LEs.
  • Stripe whole or parts of LV across multiple PVs, in a fashion similar to RAID0.
  • Mirror whole or parts of LV, in a fashion similar to RAID1.
  • Move online logical volumes between PVs.

In the 2.6 Linux kernels, the LVM is implemented in terms of the device mapper, creating virtual block devices and mapping their space onto other block devices. While LVM user-space tools manipulate these mappings and reconstruct their state from on-disk metadata upon each invocation.

Implementation

Redhat Enterprise Linux (RHEL) 5 provides an LVM user-space tool to implement logical volumes. The good news is that the tool runs on Terminal; thus, the implementation logical volumes in RHEL 5 can be performed via SSL connection remotely.

# lvm
lvm>

There are a number of LVM commands available in this tool.

  Available lvm commands:
Use 'lvm help ' for more information

dumpconfig      Dump active configuration
formats         List available metadata formats
help            Display help for commands
lvchange        Change the attributes of logical volume(s)
lvconvert       Change logical volume layout
lvcreate        Create a logical volume
lvdisplay       Display information about a logical volume
lvextend        Add space to a logical volume
lvmchange       With the device mapper, this is obsolete and does nothing.
lvmdiskscan     List devices that may be used as physical volumes
lvmsadc         Collect activity data
lvmsar          Create activity report
lvreduce        Reduce the size of a logical volume
lvremove        Remove logical volume(s) from the system
lvrename        Rename a logical volume
lvresize        Resize a logical volume
lvs             Display information about logical volumes
lvscan          List all logical volumes in all volume groups
pvchange        Change attributes of physical volume(s)
pvresize        Resize physical volume(s)
pvck            Check the consistency of physical volume(s)
pvcreate        Initialize physical volume(s) for use by LVM
pvdata          Display the on-disk metadata for physical volume(s)
pvdisplay       Display various attributes of physical volume(s)
pvmove          Move extents from one physical volume to another
pvremove        Remove LVM label(s) from physical volume(s)
pvs             Display information about physical volumes
pvscan          List all physical volumes
segtypes        List available segment types
vgcfgbackup     Backup volume group configuration(s)
vgcfgrestore    Restore volume group configuration
vgchange        Change volume group attributes
vgck            Check the consistency of volume group(s)
vgconvert       Change volume group metadata format
vgcreate        Create a volume group
vgdisplay       Display volume group information
vgexport        Unregister volume group(s) from the system
vgextend        Add physical volumes to a volume group
vgimport        Register exported volume group with system
vgmerge         Merge volume groups
vgmknodes       Create the special files for volume group devices in /dev
vgreduce        Remove physical volume(s) from a volume group
vgremove        Remove volume group(s)
vgrename        Rename a volume group
vgs             Display information about volume groups
vgscan          Search for all volume groups
vgsplit         Move physical volumes into a new volume group
version         Display software and driver version information

Example

The example was performed using HP Integrity Server with RHEL 5.1 (Tikanga). The target hard disk was /dev/cciss/c0d1 that had one partition /dev/cciss/c0d1p1. The purpose was to create a logical volume on the partition. Notice that a hard disk partition like /dev/hdb1 or /dev/sdb1 may also be a target hard disk to implement logical volume.

  • Add a new physical volumeShow the existing physical volumes.
    lvm> pvs
    PV                VG         Fmt  Attr PSize  PFree
    /dev/cciss/c0d0p2 VolGroup00 lvm2 a-   68.22G     0

    Add a new physical volume.

    lvm> pvcreate /dev/cciss/c0d1p1
    Physical volume "/dev/cciss/c0d1p1" successfully created

    Show the current physical volumes.

    lvm> pvs
    PV                VG         Fmt  Attr PSize  PFree
    /dev/cciss/c0d0p2 VolGroup00 lvm2 a-   68.22G     0
    /dev/cciss/c0d1p1            lvm2 --   67.77G 67.77G
  • Create a new volume groupShow the existing volume groups.
    lvm> vgs
    VG         #PV #LV #SN Attr   VSize   VFree
    VolGroup00   1   2   0 wz--n-  68.22G      0

    Create a new volume group.

    lvm> vgcreate VolGroup01 /dev/cciss/c0d1p1
    Volume group "VolGroup01" successfully created

    Show the existing volume groups and physical volumes.

    lvm> vgs
    VG         #PV #LV #SN Attr   VSize   VFree
    VolGroup00   1   2   0 wz--n-  68.22G      0
    VolGroup01   1   0   0 wz--n-  67.77G  67.77G
    lvm> pvs
    PV                VG         Fmt  Attr PSize  PFree
    /dev/cciss/c0d0p2 VolGroup00 lvm2 a-   68.22G     0
    /dev/cciss/c0d1p1 VolGroup01 lvm2 a-   67.77G 67.77G
  • Create a new logical volumeShow the existing logical volumes.
    lvm> lvs
    LV       VG         Attr   LSize  Origin Snap%  Move Log Copy%
    LogVol00 VolGroup00 -wi-ao 66.28G
    LogVol01 VolGroup00 -wi-ao  1.94G

    Create a new logical volume.

    lvm> lvcreate --size 30000 --name LogVol02 VolGroup01
    Rounding up size to full physical extent 29.31 GB
    Logical volume "LogVol02" created

    Show the current logical volumes and physical volumes.

    lvm> lvs
    LV       VG         Attr   LSize  Origin Snap%  Move Log Copy%
    LogVol00 VolGroup00 -wi-ao 66.28G
    LogVol01 VolGroup00 -wi-ao  1.94G
    LogVol02 VolGroup01 -wi-a- 29.31G
    lvm> pvs
    PV                VG         Fmt  Attr PSize  PFree
    /dev/cciss/c0d0p2 VolGroup00 lvm2 a-   68.22G     0
    /dev/cciss/c0d1p1 VolGroup01 lvm2 a-   67.75G 38.44G
  • Use the new logical volumeFormat the new logical volume.
    # mkfs -t ext3 /dev/VolGroup01/LogVol02
    mke2fs 1.39 (29-May-2006)
    Filesystem label=
    OS type: Linux
    Block size=4096 (log=2)
    Fragment size=4096 (log=2)
    3842720 inodes, 7684096 blocks
    384204 blocks (5.00%) reserved for the super user
    First data block=0
    Maximum filesystem blocks=4294967296
    235 block groups
    32768 blocks per group, 32768 fragments per group
    16352 inodes per group
    Superblock backups stored on blocks:
    32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,
    4096000
    
    Writing inode tables: done
    Creating journal (32768 blocks): done
    Writing superblocks and filesystem accounting information: done
    
    This filesystem will be automatically checked every 38 mounts or
    180 days, whichever comes first.  Use tune2fs -c or -i to override.

    Mount the new logical volume.

    # mount /dev/VolGroup01/LogVol02 /mnt/storage01
    # df
    Filesystem           1K-blocks      Used Available Use% Mounted on
    /dev/mapper/VolGroup00-LogVol00
    67323516   3138844  60709628   5% /
    /dev/cciss/c0d0p1       102182      9230     92952  10% /boot/efi
    tmpfs                  1998224         0   1998224   0% /dev/shm
    /dev/mapper/VolGroup01-LogVol02
    30254032    176200  28541016   1% /mnt/storage01
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