Veritas Volume Manager 5.0 Troubleshooting Guide Guide (September 2006)

20 Recovery from hardware failure
Failures on RAID-5 volumes
■ Parity resynchronization
■ Log plex recovery
■ Stale subdisk recovery
Parity resynchronization and stale subdisk recovery are typically performed when the
RAID-5 volume is started, or shortly after the system boots. They can also be performed
by running the vxrecover command.
For more information on starting RAID-5 volumes, see “Starting RAID-5 volumes” on
page 23.
If hot-relocation is enabled at the time of a disk failure, system administrator intervention
is not required unless no suitable disk space is available for relocation. Hot-relocation is
triggered by the failure and the system administrator is notified of the failure by electronic
mail.
Hot relocation automatically attempts to relocate the subdisks of a failing RAID-5 plex.
After any relocation takes place, the hot-relocation daemon (vxrelocd) also initiates a
parity resynchronization.
In the case of a failing RAID-5 log plex, relocation occurs only if the log plex is mirrored;
the vxrelocd daemon then initiates a mirror resynchronization to recreate the RAID-5
log plex. If hot-relocation is disabled at the time of a failure, the system administrator may
need to initiate a resynchronization or recovery.
Note: Following severe hardware failure of several disks or other related subsystems
underlying a RAID-5 plex, it may be impossible to recover the volume using the methods
described in this chapter. In this case, remove the volume, recreate it on hardware that is
functioning correctly, and restore the contents of the volume from a backup.
Parity resynchronization
In most cases, a RAID-5 array does not have stale parity. Stale parity only occurs after all
RAID-5 log plexes for the RAID-5 volume have failed, and then only if there is a system
failure. Even if a RAID-5 volume has stale parity, it is usually repaired as part of the
volume start process.
If a volume without valid RAID-5 logs is started and the process is killed before the
volume is resynchronized, the result is an active volume with stale parity. For an example
of the output of the
vxprint -ht command, see the following example for a stale RAID-
5 volume:
V NAME RVG/VSET/COKSTATE STATE LENGTH READPOL PREFPLEX UTYPE
PL NAME VOLUME KSTATE STATE LENGTH LAYOUT NCOL/WID MODE
SD NAME PLEX DISK DISKOFFS LENGTH [COL/]OFF DEVICE MODE
SV NAME PLEX VOLNAME NVOLLAYR LENGTH [COL/]OFF AM/NM MODE
...