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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 |
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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 |
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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 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.
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