VERITAS Volume Manager 3.5 Administrator's Guide (September 2002)

Chapter 1, Understanding VERITAS Volume Manager
Volume Layouts in VxVM
29
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.
Thefigure,“Incomplete Write,” illustrates a RAID-5volumeconfiguredacrossthreedisks
(A, Band C). In this volume,recovery of disk B’scorrupted datadepends 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 thearray. Inthis way, thelog canbe replayed, causing the data andparity updatesto be
completed before the reconstruction of the failed drive takes place.
Logs are associated with a RAID-5volume by being attached as logplexes. More thanone
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 198 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