Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

57Understanding Veritas Volume Manager
Dirty region logging
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 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 a
tunable as detailed in the description of voldrl_max_seq_dirty in “Tunable
parameters” on page 463.
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 traditional DRL
logging to a mirrored volume” on page 273, and “Preparing a volume for DRL and instant
snapshots” on page 267.
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. If supported by the database vendor, the SmartSync feature uses an extended
interface between VxVM volumes and the database software to avoid unnecessary work
during mirror resynchronization. For example, Oracle
®
automatically takes advantage of
SmartSync to perform database resynchronization when it is available.