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An uncorrectable I/O error occurs when disk failure, cabling or other problems cause the data on a disk to become unavailable. For a RAID-5 volume, this means that a subdisk becomes unavailable. 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/VSET/CO KSTATE 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
v r5vol - ENABLED DEGRADED 204800 RAID - raid5
pl r5vol-01 r5vol ENABLED ACTIVE 204800 RAID 3/16 RW
sd disk01-01 r5vol-01 disk01 0 102400 0/0 c2t9d0 ENA
sd disk02-01 r5vol-01 disk02 0 102400 1/0 c2t10d0 dS
sd disk03-01 r5vol-01 disk03 0 102400 2/0 c2t11d0 ENA
pl r5vol-02 r5vol ENABLED LOG 1440 CONCAT - RW
sd disk04-01 r5vol-02 disk04 0 1440 0 c2t12d0 ENA
pl r5vol-03 r5vol ENABLED LOG 1440 CONCAT - RW
sd disk05-01 r5vol-03 disk05 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.
A disk containing a RAID-5 log plex can also fail. The failure of a single RAID-5 log plex has no direct effect on the operation of a volume provided that the RAID-5 log is mirrored. However, loss of all RAID-5 log plexes in a volume makes it vulnerable to a complete failure. In the output of the vxprint -ht command, failure within a RAID-5 log plex is indicated by the plex state being shown as BADLOG rather than LOG.
In the following example, the RAID-5 log plex r5vol-02 has failed:
V NAME RVG/VSET/CO KSTATE 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
v r5vol - ENABLED ACTIVE 204800 RAID - raid5
pl r5vol-01 r5vol ENABLED ACTIVE 204800 RAID 3/16 RW
sd disk01-01 r5vol-01 disk01 0 102400 0/0 c2t9d0 ENA
sd disk02-01 r5vol-01 disk02 0 102400 1/0 c2t10d0 ENA
sd disk03-01 r5vol-01 disk03 0 102400 2/0 c2t11d0 ENA
pl r5vol-02 r5vol DISABLED BADLOG 1440 CONCAT - RW
sd disk04-01 r5vol-02 disk04 0 1440 0 c2t12d0 ENA
pl r5vol-03 r5vol ENABLED LOG 1440 CONCAT - RW
sd disk05-01 r5vol-12 disk05 0 1440 0 c2t14d0 ENA