Dirty Region Logging (DRL)

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.

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.

For details of how to configure DRL and sequential DRL, see Adding Traditional DRL Logging to a Mirrored Volume, and Preparing a Volume for DRL and Instant Snapshots.