Veritas Volume Manager 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

Dirty Region Logging (DRL)
42 VERITAS Volume Manager Administrator’s Guide
The process of resynchronization can impact system performance. The recovery process
reduces some of this impact by spreading the recoveries to avoid stressing a specific disk
or controller.
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 (DRL)”
on page 42 and “FastResync” on page 49 for more information.
For raw volumes used by database applications, the SmartSync Recovery Accelerator can
be used if this is supported by the database vendor (see “SmartSync Recovery
Accelerator” on page 44).
Dirty Region Logging (DRL)
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 Vo lum e Ver sion ing” on page 51.
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.