VERITAS Volume Manager 3.1 Administrator's Guide

Recovery
Failures and RAID-5 Volumes
Chapter 8370
failure. The process of resynchronization consists of reading that data
and parity from the logs and writing it to the appropriate areas of the
RAID-5 volume. This greatly reduces the amount of time needed for a
resynchronization of data and parity. It also means that the volume
never becomes truly stale. The data and parity for all stripes in the
volume are known at all times, so the failure of a single disk cannot
result in the loss of the data within the volume.
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
vxprint, as shown in the following display:
VNAMERVGKSTATESTATELENGTHUSETYPEPREFPLEXRDPOL
PLNAMEVOLUMEKSTATESTATELENGTHLAYOUTNCOL/WIDMODE
SDNAMEPLEXDISKDISKOFFSLENGTH[COL/]OFFDEVICEMODE
SVNAMEPLEXVOLNAMENVOLLAYRLENGTH[COL/]OFFAM/NMMODE
vr5vol-ENABLEDDEGRADED204800raid5-RAID
plr5vol-01r5volENABLEDACTIVE204800RAID3/16RW
sddisk01-01r5vol-01disk0101024000/0c2t9d0ENA
sddisk02-01r5vol-01disk0201024001/0c2t10d0ENA
sddisk03-01r5vol-01disk0301024002/0c2t11d0ENA
plr5vol-02r5volENABLEDLOG1440CONCAT-RW
sddisk04-01r5vol-02disk04014400c2t12d0ENA