Dirty Region Logging (DRL)

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 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 can be 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 the tunable voldrl_max_seq_dirty as described in the description of “voldrl_max_seq_dirty”.

	NOTE: DRL adds a small I/O overhead for most write access patterns.

For details of how to configure DRL and sequential DRL, see “Adding DRL Logging to a Mirrored Volume”.