Planning For Recovery - HP -UX 11i Administrator's Manual

Planning for Recovery

Flexibility in configuration, one of the major benefits of LVM, can also be a source of problems in
recovery. The following are guidelines to help create a configuration that minimizes recovery time:
Keep the number of disks in the root volume group to a minimum; HP recommends using three
disks, even if the root volume group is mirrored.
Root volume groups with many disks make reinstallation difficult because of the complexity of
recovering LVM configurations of accessory disks within the root volume group.
A small root volume group is quickly recovered. In some cases, you can reinstall a minimal
system, restore a backup, and be back online within three hours of diagnosis and replacement
of hardware. Another benefit is that exact match to the previous root disk layout is not required.
Three disks in the root volume group are better than two, because of quorum restrictions. With
a two-disk root volume group, the loss of one disk can require you to override quorum to
activate the volume group; if you must reboot to replace the disk, overriding quorum requires
you to interrupt the boot process. If you have three disks in the volume group and they are
isolated from each other such that a hardware failure only affects one of them, then failure of
only one disk enables the system to maintain quorum.
There are two reasons to expand the root volume group beyond a minimal size.
◦ A very small root disk.
In this case, HP recommends migrating or installing to a larger disk.
◦ Providing for dump-to-swap for large memory systems.
Swap volumes targeted for dump must be in the root volume group. A better solution is
to configure an extra dedicated disk for dump up front.
Do not use network-based backup programs such as Omniback or Networker for basic root
volume group backups. The complexity associated with these utilities can significantly delay
the resumption of processing.
HP recommends using Ignite-UX and the make_net_recovery command to back up and
recover your root volume group.
Create adequate documentation.
Output from ioscan -kf, vgcfgrestore -lv for all groups and vgscan -pv, and
lvlnboot -v are very minimal requirements. Recovery from almost any problem is possible
if these and output from vgdisplay -v for all groups and lvdisplay -v for all volumes
are available. Extent mappings are critical to recovery of LVM volumes with corrupted headers.
Additionally, output from pvdisplay -v for all physical volumes, although not as important,
provides complete volume group information. Hard copy is not required or even necessarily
practical, but accessibility during recovery is important and should be planned for.
Many small groups are better than fewer large groups when configuring auxiliary volume
groups.
Reloading dozens of gigabytes of data because one disk is missing out of a group after a
root disk rebuild can be difficult. Also the scope of a catastrophic single disk failure within a
group is minimized with many small groups.
Size implies complexity. The larger the number of disks in a single group, the more chances
an administrator has to create an error that affects the entire group. It is simpler to identify
and import smaller groups if necessary. It is also simpler to conceptualize and map smaller
groups when required.
34 Configuring LVM