Veritas Volume Manager 5.1 SP1 Administrator"s Guide (5900-1506, April 2011)

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.
Note: DRL adds a small I/O overhead for most write access patterns. This overhead
is reduced by using SmartSync.
If a version 20 DCO volume is associated with a volume, a portion of the DCO
volume can be used to store the DRL log. There is no need to create a separate
DRL log for a volume which has a version 20 DCO volume.
See “DCO volume versioning” on page 65.
Log subdisks and plexes
DRL log subdisks 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.
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. 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 a tunable as detailed in the description of voldrl_max_seq_dirty.
See “Tunable parameters for VxVM” on page 496.
See “Adding traditional DRL logging to a mirrored volume” on page 365.
Understanding Veritas Volume Manager
Dirty region logging
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