Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

47Understanding Veritas Volume Manager
Volume layouts in VxVM
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.
Figure 1-24 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.
Figure 1-24 Incomplete write to a RAID-5 volume
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 275 for information on how to add a RAID-5 log to a
RAID-5 volume.
Layered volumes
A layered volume is a virtual Veritas Volume Manager object that is built on top of other
volumes. The layered volume structure tolerates failure better and has greater redundancy
than the standard volume structure. For example, in a striped-mirror layered volume, each
Completed
Corrupted data
Incomplete
Disk A Disk B Disk C
data write parity write