Veritas Volume Manager 5.0.1 Administrator's Guide, HP-UX 11i v3, First Edition, November 2009
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 shows a RAID-5 volume configured across three disks (A, B and C).
Figure 1-24
Incomplete write to a RAID-5 volume
Disk A Disk CDisk B
Completed
data write
Corrupted data
Incomplete
parity write
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.
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 328.
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 mirror (plex) covers a smaller area of storage
space, so recovery is quicker than with a standard mirrored volume.
Understanding Veritas Volume Manager
Volume layouts in VxVM
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