LRU write

LRU writes are performed by page cleaners rather than by sqlexec threads. The database server performs LRU writes as background writes that typically occur when the percentage of dirty buffers exceeds the percent that is specified for lru_max_dirty in the BUFFERPOOL configuration parameter.

A foreground write can trigger an LRU write. When a foreground write occurs, the sqlexec thread that performed the write alerts a page-cleaner to wake up and clean the LRU for which it performed the foreground write.

In an appropriately tuned system, page cleaners ensure that enough unmodified buffer pages are available for storing pages to be read from disk. Thus, sqlexec threads that perform a query are not required to flush a page to disk before they read in the disk pages required by the query. This condition can result in significant performance gains for queries that do not make use of foreground writes.

LRU writes are preferred over foreground writes because page-cleaner threads perform buffer writes much more efficiently than sqlexec threads do. To monitor both types of writes, use onstat -F.

Copyright© 2018 HCL Technologies Limited