VERITAS Volume Manager 3.5 Troubleshooting Guide (August 2002)
Chapter 1, Recovery from Hardware Failure
Failures on RAID-5 Volumes
7
Disk Failures
Disk failures can cause the data on a disk to become unavailable. In terms of a RAID-5
volume, this means that a subdisk becomes unavailable.
This can occur due to an uncorrectable I/O error during a write to the disk. The I/O error
can cause the subdisk to be detached from the array or a disk being unavailable when the
system is booted (for example, from a cabling problem or by having a drive powered
down).
When this occurs, the subdisk cannot be used to hold data and is considered stale and
detached. If the underlying disk becomes available or is replaced, the subdisk is still
considered stale and is not used.
If an attempt is made to read data contained on a stale subdisk, the data is reconstructed
from data on all other stripe units in the stripe. This operation is called a
reconstructing-read. This is a more expensive operation than simply reading the data and
can result in degraded read performance. When a RAID-5 volume has stale subdisks, it is
considered to be in degraded mode.
A RAID-5 volume in degraded mode can be recognized from the output of the vxprint
-ht command as shown in the following display:
V NAME RVG KSTATE STATE LENGTH READPOL PREFPLEX UTYPE
PL NAME VOLUME KSTATE STATE LENGTH LAYOUT NCOL/WID MODE
SD NAME PLEX DISK DISKOFFSLENGTH [COL/]OFF DEVICE MODE
SV NAME PLEX VOLNAME NVOLLAYRLENGTH [COL/]OFF AM/NM MODE
...
v r5vol - ENABLED DEGRADED204800 RAID - raid5
pl r5vol-01 r5vol ENABLED ACTIVE 204800 RAID 3/16 RW
sd disk01-01 r5vol-01disk01 0 102400 0/0 c2t9d0 ENA
sd disk02-01 r5vol-01disk02 0 102400 1/0 c2t10d0 dS
sd disk03-01 r5vol-01disk03 0 102400 2/0 c2t11d0 ENA
pl r5vol-02 r5vol ENABLED LOG 1440 CONCAT - RW
sd disk04-01 r5vol-02disk04 0 1440 0 c2t12d0 ENA
pl r5vol-03 r5vol ENABLED LOG 1440 CONCAT - RW
sd disk05-01 r5vol-03disk05 0 1440 0 c2t14d0 ENA
The volume r5vol is in degraded mode, as shown by the volume state, which is listed as
DEGRADED. The failed subdisk is disk02-01, as shown by the MODE flags; d indicates
that the subdisk is detached, and S indicates that the subdisk’s contents are stale.
Note Do not run the vxr5check command on a RAID-5 volume that is in degraded
mode.
A disk containing a RAID-5 log plex can also fail. The failure of a single RAID-5 log plex
has no directeffecton the operation ofa volume providedthat theRAID-5 logis mirrored.
However, loss of all RAID-5 log plexes in a volume makes it vulnerable to a complete