VERITAS Volume Manager 3.1 Administrator's Guide

Recovery
Miscellaneous RAID-5 Operations
Chapter 8 379
Figure 8-1 Invalid RAID-5 Volume
This example shows four stripes in the RAID-5 array. All parity is stale
and subdisk disk05-00 has failed. This makes stripes X and Y unusable
because two failures have occurred within those stripes.
This qualifies as two failures within a stripe and prevents the use of the
volume. In this case, the output display from the vxvol start command
is as follows:
vxvm:vxvol: ERROR: Volume r5vol is not startable; some
subdisks are
unusable and the parity is stale.
This situation can be avoided by always using two or more RAID-5 log
plexes in RAID-5 volumes. RAID-5 log plexes prevent the parity within
the volume from becoming stale which prevents this situation (see
“System Failures” for details).
Forcibly Starting RAID-5 Volumes
You can start a volume even if subdisks are marked as stale. For
example, if a stopped volume has stale parity and no RAID-5 logs and a
disk becomes detached and then reattached.
The subdisk is considered stale even though the data is not out of date
(because the volume was in use when the subdisk was unavailable) and
the RAID-5 volume is considered invalid. To prevent this case, always
disk00-00
disk02-00
disk01-00
disk03-00
disk05-00
disk04-00
Parity
Data
DataParity
Parity
Parity
Data
Data
Data
Data
Data
Data
RAID-5 Plex
W
X
Y
Z