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

Dirty Region Logging (DRL)
40 VERITAS Volume Manager Administrator’s Guide
Dirty Region Logging (DRL)
Note You may need an additional license to use this feature.
Dirty region logging (DRL), if enabled, speeds recovery of mirrored volumes after a
system crash. 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 those portions of the volume
that need to be recovered.
If DRL is not used and a system failureoccurs, allmirrors of the volumes mustbe 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.
Dirty Region Logs
DRL logically divides a volume into a set of consecutive regions, and maintains a dirty
region log on disk where each region is represented by one status bit. Before any data is
written to any region, DRL synchronously marks the corresponding bit in the log as dirty
if it was previously clean. The log is only used to represent regions of the volume on
which writes are pending. Once a write has been completed, the dirty bit for a region is
not cleared immediately. If another write to the same region occurs, this means it is not
necessary to write the log to the disk before the write operation can occur. The bit remains
marked as dirty until the corresponding volume region becomes the least recently
accessed for writing.
On restarting a system after a crash,VxVM recoversonly those regions of the volumethat
are marked as dirty in the dirty region log.
Log subdisks
Log subdisks are used to store the dirty region log of a mirrored 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 is referred to as a log plex.
The log subdisk can also be associated with a regular plex that contains 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.