System Recovery Week: Using LVM In Rescue Mode

Logical Volume Management (LVM) is a powerful storage system layer which abstracts the logical view of storage from the actual physical layout. It is automatically configured in the default Fedora storage configuration. LVM enables you to grow and shrink volumes, add storage from new devices to existing volumes, and migrate volumes between storage devices, all without taking the system offline. In fact, the system-config-lvm package provides a convenient way to perform all of these operations using a graphical interface.

However, you can't (yet) shrink a filesystem while it is mounted. This isn't a limitation of LVM itself, but of most filesystems, including ext3. To shrink a partition that is always mounted when the system is running -- such as the root filesystem -- it's necessary to boot from another medium, which is where rescue mode comes in (see yesterday's article). It may also be necessary to use rescue mode to recover from some particularly nasty corruption or misconfiguration issues.

To do LVM and filesystem-resizing work within the Fedora rescue mode, boot the rescue mode from disc as usual, but select Skip when the system offers to mount your hard-disk partitions. Unfortunately, this will mean that your volume groups will not be detected or activated, so you will need to do that manually.

The commands normally used for logical volume management such as vgdisplay, pvcreate, and lvreduce are actually symbolic links to a single executable named lvm. These symbolic links are not available in rescue mode, so you must explicitly use the lvm command followed by the operation you wish to perform: if you wish to do a vgscan, for example, enter the command lvm vgscan.

Therefore, to discover and activate all volume groups, you must execute these commands:

sh-3.2# lvm vgscan
Reading all physical volumes. This may take a while...
Found volume group "VolGroup00" using metadata type lvm2
sh-3.2# lvm vgchange -ay
2 logical volume(s) in volume group "VolGroup00" now active

You can display information about the logical volumes using lvm lvs (or lvm lvdisplay for a more verbose display):

sh-3.2# lvm lvs
LV VG Attr LSize Origin Snap% Move Log Copy%
LogVol00 VolGroup00 -wi-a- 28.66G
LogVol01 VolGroup00 -wi-a- 992.00M

Note that the LV and VG names are not very descriptive -- which is why it's a good idea to override the default names during system installation. In this case, we know the approximate size of the LV containing the root filesystem, so we can determine that it is LogVol00 in VolGroup00.

Armed with this information, you can now check (fsck) the filesystem, shrink the filesystem within that logical volume, and then shrink the LV. Because of the potential for rounding errors, it's best to shrink the filesystem to a size slightly smaller than the new LV size, resize the LV, and then grow the filesystem to fully fill the LV:

sh-3.2# e2fsck -f /dev/VolGroup00/LogVol00
e2fsck 1.40.2 (12-Jul-2007)
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking refernce counts
Pass 5: Checking group summary information
/dev/VolGroup00/LogVol00: 122967/7514560 files (0.9% non-contiguous), 1113272/7512064 blocks

sh-3.2# resize2fs /dev/VolGroup00/LogVol00 19G
resize2fs 1.40.2 (12-Jul-2007)
Resizing the filesystem on /dev/VolGroup00/LogVol00 to 4980736 (4k) blocks.
The filesystem on /dev/VolGroup00/LogVol00 is now 4980736 blocks long.

sh-3.2# lvm lvresize VolGroup00/LogVol00 --size 20G
WARNING: Reducing active logical volume to 20.00 GB
THIS MAY DESTROY YOUR DATA (filesystem etc.)
Do you really want to reduce LogVol00? [y/n]: y
Reducing logical volume LogVol00 to 20.00 GB
Logical volume LogVol00 successfully resized

sh-3.2# resize2fs /dev/VolGroup00/LogVol00
resize2fs 1.40.2 (12-Jul-2007)
Resizing the filesystem on /dev/VolGroup00/LogVol00 to 5242880 (4k) blocks.
The filesystem on /dev/VolGroup00/LogVol00 is now 5242880 blocks long.

Note that the operations following the first resize2fs do not need to be performed in rescue mode -- you can reboot the system at that point and continue after the system has booted normally. Earlier versions of the Fedora rescue mode may not set up the symbolic link /dev/VolumeGroup/LogicalVolume, so you may need to refer to /dev/mapper/VolumeGroup-LogicalVolume (e.g., use /dev/mapper/VolGroup00-LogVol00 in place of /dev/VolGroup00/LogVol00).