Monday, August 17, 2009

Reducing a logical volume

Logical volumes can be reduced in size as well as increased. However, it is very important to remember to reduce the size of the file system or whatever is residing in the volume before shrinking the volume itself, otherwise you risk losing data.

  1. ext2
    If you are using LVM 1 with ext2 as the file system then you can use the e2fsadm command mentioned earlier to take care of both the file system and volume resizing as follows:

    # umount /home
    # e2fsadm -L-1G /dev/myvg/homevol
    # mount /home
    WarningLVM 2 Caveat
    There is currently no e2fsadm equivalent for LVM 2 and the e2fsadm that ships with LVM 1 does not work with LVM 2.
    If you prefer to do this manually you must know the new size of the volume in blocks and use the following commands:

    # umount /home
    # resize2fs /dev/myvg/homevol 524288
    # lvreduce -L-1G /dev/myvg/homevol
    # mount /home

  2. reiserfs
    Reiserfs seems to prefer to be unmounted when shrinking

    # umount /home
    # resize_reiserfs -s-1G /dev/myvg/homevol
    # lvreduce -L-1G /dev/myvg/homevol
    # mount -treiserfs /dev/myvg/homevol /home

  3. xfs
    There is no way to shrink XFS file systems.

  4. jfs
    There is no way to shrink JFS file systems.

    Differences between LVM1 and LVM2

The new release of LVM, LVM 2, is available only on Red Hat Enterprise Linux 4 and later kernels. It is upwardly compatible with LVM 1 and retains the same command line interface structure. However it uses a new, more scalable and resilient metadata structure that allows for transactional metadata updates (that allow quick recovery after server failures), very large numbers of devices, and clustering. For Enterprise Linux servers deployed in mission-critical environments that require high availability, LVM2 is the right choice for Linux volume management. Table 1. A comparison of LVM 1 and LVM 2 summarizes the differences between LVM1 and LVM2 in features, kernel support, and other areas.

Features LVM1 LVM2
RHEL AS 2.1 support No No
RHEL 3 support Yes No
RHEL 4 support No Yes
Transactional metadata for fast recovery No Yes
Shared volume mounts with GFS No Yes
Cluster Suite failover supported Yes Yes
Striped volume expansion No Yes
Max number PVs, LVs 256 PVs, 256 LVs 2**32 PVs, 2**32 LVs
Max device size 2 Terabytes 8 Exabytes (64-bit CPUs)
Volume mirroring support No Yes, in Fall 2005

Table 1. A comparison of LVM 1 and LVM 2

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