VERITAS Volume Manager 3.1 Administrator's Guide

Volume Manager Operations
Dirty Region Logging
Chapter 3112
Dirty Region Logging
NOTE You must license the VERITAS Volume Manager product to use this
feature.
Dirty Region Logging (DRL) is an optional property of a volume, used to
provide a speedy recovery of mirrored volumes after a system failure.
DRL keeps track of the regions that have changed due to I/O writes to a
mirrored volume. DRL uses this information to recover only the portions
of the volume that need to be recovered.
If DRL is not used and a system failure occurs, all mirrors of the volumes
must be restored to a consistent state. Restoration is done by copying the
full contents of the volume between its mirrors. This process can be
lengthy and I/O intensive. It may also be necessary to recover the areas
of volumes that are already consistent.
DRL logically divides a volume into a set of consecutive regions. It keeps
track of volume regions that are being written to. A dirty region log is
maintained that contains a status bit representing each region of the
volume. For any write operation to the volume, the regions being written
are marked dirty in the log before the data is written. If a write causes a
log region to become dirty when it was previously clean, the log is
synchronously written to disk before the write operation can occur. On
system restart, the Volume Manager recovers only those regions of the
volume that are marked as dirty in the dirty region log.
Log subdisks are used to store the dirty region log of a volume that has
DRL enabled. A volume with DRL has at least one log subdisk; multiple
log subdisks can be used to mirror the dirty region log. Each log subdisk
is associated with one plex of the volume. Only one log subdisk can exist
per plex. If the plex contains only a log subdisk and no data subdisks,
that plex can be referred to as a log plex.
The log subdisk can also be associated with a regular plex containing
data subdisks. In that case, the log subdisk risks becoming unavailable if
the plex must be detached due to the failure of one of its data subdisks.
If the vxassist command is used to create a dirty region log, it creates a
log plex containing a single log subdisk by default. A dirty region log can