VERITAS Volume Manager 4.1 Administrator's Guide
Understanding VERITAS Volume Manager
Dirty Region Logging (DRL)
Chapter 146
Dirty Region Logging (DRL)
NOTE You may need an additional 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 dirty region log on disk where each region is represented by
one status bit. Before any data is written to any region, DRL
synchronously marks the corresponding bit in the log as dirty if it was
previously clean. The log is only used to represent regions of the volume
on which writes are pending. Once a write has been completed, the dirty
bit for a region is not cleared immediately. If another write to the same
region occurs, this means it is not necessary to write the log to the disk
before the write operation can occur. The bit remains marked as dirty
until the corresponding volume region becomes the least recently
accessed for writing.
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.
Log subdisks
Log subdisks are used to 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