VERITAS Volume Manager 3.1 Administrator's Guide

Volume Manager Operations
Dirty Region Logging
Chapter 3 113
also be created manually by creating a log subdisk and associating it
with a plex. Then the plex can contain both a log subdisk and data
subdisks.
Only a limited number of bits can be marked dirty in the log at any time.
The dirty bit for a region is not cleared immediately after writing the
data to the region. Instead, it remains marked as dirty until the
corresponding volume region becomes the least recently used. If a bit for
a given region is already marked dirty and another write to the same
region occurs, it is not necessary to write the log to the disk before the
write operation can occur.
Some volumes, such as those used for Oracle replay logs, are written
sequentially and do not benefit from this lazy cleaning of the DRL bits.
For these volumes, sequential DRL can be used to further restrict the
number of dirty bits and speed up recovery. The number of dirty bits
allowed for sequential DRL is restricted by the tunable
voldrl_max_dirty. Using sequential DRL on volumes that are written
sequentially may severely impact I/O throughput.
NOTE DRL adds a small I/O overhead for most write access patterns.