Veritas Volume Manager 5.0.1 Troubleshooting Guide, HP-UX 11i v3, First Edition, November 2009

synchronization occurs because the status of writes that were outstanding at the
time of the failure cannot be determined.
If a loss of sync occurs while a RAID-5 volume is being accessed, the volume is
described as having stale parity. The parity must then be reconstructed by reading
all the non-parity columns within each stripe, recalculating the parity, and writing
out the parity stripe unit in the stripe. This must be done for every stripe in the
volume, so it can take a long time to complete.
Warning: While the resynchronization of a RAID-5 volume without log plexes is
being performed, any failure of a disk within the volume causes its data to be lost.
Besides the vulnerability to failure, the resynchronization process can tax the
system resources and slow down system operation.
RAID-5 logs reduce the damage that can be caused by system failures, because
they maintain a copy of the data being written at the time of the 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
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/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
19Recovering from hardware failure
Failures on RAID-5 volumes