Veritas Volume Manager 5.0 Troubleshooting Guide, HP-UX 11i v3, First Edition, May 2008

15Recovery from hardware failure
Failures on RAID-5 volumes
You can use the command vxreattach -c to check whether reattachment is
possible, without performing the operation. Instead, it displays the disk group
and disk media name where the disk can be reattached.
See the vxreattach(1M) manual page for more information on the
vxreattach
command.
Failures on RAID-5 volumes
Failures are seen in two varieties: system failures and disk failures. A system
failure means that the system has abruptly ceased to operate due to an
operating system panic or power failure. Disk failures imply that the data on
some number of disks has become unavailable due to a system failure (such as a
head crash, electronics failure on disk, or disk controller failure).
System failures
RAID-5 volumes are designed to remain available with a minimum of disk space
overhead, if there are disk failures. However, many forms of RAID-5 can have
data loss after a system failure. Data loss occurs because a system failure causes
the data and parity in the RAID-5 volume to become unsynchronized. Loss of
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.
Caution: 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.