VERITAS Volume Manager 4.1 Administrator's Guide

Understanding VERITAS Volume Manager
Volume Layouts in VxVM
Chapter 1 35
failure, the data for each stripe can be restored by XORing the contents
of the remaining columns data stripe units against their respective
parity stripe units.
For example, if a disk corresponding to the whole or part of the far left
column fails, the volume is placed in a degraded mode. While in degraded
mode, the data from the failed column can be recreated by XORing stripe
units 1-3 against parity stripe unit P0 to recreate stripe unit 0, then
XORing stripe units 4, 6, and 7 against parity stripe unit P1 to recreate
stripe unit 5, and so on.
NOTE Failure of more than one column in a RAID-5 plex detaches the volume.
The volume is no longer allowed to satisfy read or write requests. Once
the failed columns have been recovered, it may be necessary to recover
user data from backups.
RAID-5 Logging
Logging is used to prevent corruption of data during recovery by
immediately recording changes to data and parity to a log area on a
persistent device such as a volume on disk or in non-volatile RAM. The
new data and parity are then written to the disks.
Without logging, it is possible for data not involved in any active writes
to be lost or silently corrupted if both a disk in a RAID-5 volume and the
system fail. If this double-failure occurs, there is no way of knowing if the
data being written to the data portions of the disks or the parity being
written to the parity portions have actually been written. Therefore, the
recovery of the corrupted disk may be corrupted itself.
The figure, “Incomplete Write,” illustrates a RAID-5 volume configured
across three disks (A, B and C). In this volume, recovery of disk B’s
corrupted data depends on disk A’s data and disk C’s parity both being