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

56 Understanding Veritas Volume Manager
Dirty region logging
For large volumes or for a large number of volumes, the resynchronization process can
take time. These effects can be addressed by using dirty region logging (DRL) and
FastResync (fast mirror resynchronization) for mirrored volumes, or by ensuring that
RAID-5 volumes have valid RAID-5 logs. See the sections “Dirty region logging” on
page 56 and “FastResync” on page 62 for more information.
For raw volumes used by database applications, the SmartSync feature can be used if this
is supported by the database vendor (see “SmartSync recovery accelerator” on page 57).
Dirty region logging
Note: 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. For more information, see “DCO
volume versioning” on page 64.
You need a full 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 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. 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 log on
disk where each region is represented by a status bit. This log records regions of a volume
for which writes are pending. Before data is written to a region, DRL synchronously
marks the corresponding status bit in the log as dirty. To enhance performance, the log bit
remains set to dirty until the region becomes the least recently accessed for writes. This
allows writes to the same region to be written immediately to disk if the region’s log bit is
set to dirty.
On restarting a system after a crash, VxVM recovers only those regions of the volume that
are marked as dirty in the dirty region log.