Veritas Volume Manager 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

Chapter 1, Understanding VERITAS Volume Manager
Volume Layouts in VxVM
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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 complete. However, only the data write to disk A is
complete. The parity write to disk C is incomplete, which would cause the data on disk B
to be reconstructed incorrectly.
Incomplete Write
This failure can be avoided by logging all data and parity writes before committing them
to the array. In this way, the log can be replayed, causing the data and parity updates to be
completed before the reconstruction of the failed drive takes place.
Logs are associated with a RAID-5 volume by being attached as log plexes. More than one
log plex can exist for each RAID-5 volume, in which case the log areas are mirrored.
See “Adding a RAID-5 Log” on page 243 for information on how to add a RAID-5 log to a
RAID-5 volume.
Completed
Corrupted Data
Incomplete
Disk A Disk B Disk C
Data Write Parity Write