VERITAS Volume Manager 4.1 Administrator's Guide

Understanding VERITAS Volume Manager
Dirty Region Logging (DRL)
Chapter 1 47
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.
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
also be set up manually by creating a log subdisk and associating it with
a plex. The plex then contains both a log and data subdisks.
Sequential DRL
Some volumes, such as those that are used for database replay logs, are
written sequentially and do not benefit from delayed cleaning of the DRL
bits. For these volumes, sequential DRL can be used to limit the number
of dirty regions. This allows for faster recovery should a crash occur.
However, if applied to volumes that are written to randomly, sequential
DRL can be a performance bottleneck as it limits the number of parallel
writes that can be carried out.
The maximum number of dirty regions allowed for sequential DRL is
controlled by the tunable voldrl_max_seq_dirty as described in the
description of “voldrl_max_seq_dirty” on page 412.
NOTE DRL adds a small I/O overhead for most write access patterns.
For details of how to configure DRL and sequential DRL, see “Adding
DRL Logging to a Mirrored Volume” on page 283.