Veritas Volume Manager 5.0.1 Administrator's Guide, HP-UX 11i v3, First Edition, November 2009

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 “DMP tunable parameters ” on page 540.
See “Adding traditional DRL logging to a mirrored volume” on page 326.
See “Preparing a volume for DRL and instant snapshots” on page 318.
SmartSync recovery accelerator
The SmartSync feature of Veritas Volume Manager increases the availability of
mirrored volumes by only resynchronizing changed data. (The process of
resynchronizing mirrored databases is also sometimes referred to as resilvering.)
SmartSync reduces the time required to restore consistency, freeing more I/O
bandwidth for business-critical applications. SmartSync uses an extended interface
between VxVM volumes, VxFS file systems, and the Oracle database to avoid
unnecessary work during mirror resynchronization and to reduce the I/O overhead
of the DRL. For example, Oracle® automatically takes advantage of SmartSync to
perform database resynchronization when it is available.
Note: To use SmartSync with volumes that contain file systems, see the section
on the Oracle Resilvering feature of Veritas File System (VxFS).
61Understanding Veritas Volume Manager
Dirty region logging