Nigel Noble's Oracle Blog

05/07/2010, KEEP pool / Serial Direct Read

Filed under:, 11gR1, Performance — Nigel Noble @ 11:36 am

Jonathan Lewis made reference to a 11g bug related to using a KEEP POOL in his note Not KEEPing.  Oracle 11g introduced a new feature called adaptive serial direct path reads which allows large “Full Scan” disk reads  to be read using “direct path reads” rather than through the buffer cache. In many cases “direct IO” can give significant increase in performance when reading data for first time (from disk), however can be significantly slower if your subsequent queries could have been serviced from the buffer cache. The bug Jonathan references (Bug 8897574) causes problems if you assign any large object to a KEEP POOL because by default, 11g would read large objects using the new direct IO feature and avoid  ever placing the object in the KEEP POOL. The whole point of using the KEEP POOL is to identify objects you do want to protect and keep in a cache. 

The patchset has also back-ported the same direct read feature which is new to 11g although I don’t know if the rules are the same as 11g. The site where I work makes significant use of KEEP pools and also has spent some time investigating aspects relating to serial direct IO vs. buffer cache IO. 

 I want to use this blog entry to explore a number of related issues but also demonstrate that the 11g bug Jonathan identified seems to also exists in patchset (and 11gR2). This blog item will cover:

  • Brief reference relating to the 11g “adaptive serial direct path read”
  • The implementation and how to switch it on
  • demonstration showing the relative difference for different types of IO vs read from cache
  • demonstration which shows the KEEP pool bug also exists (but not by default)
  • Some real life comparison figures of disk reads via “direct path read” and via “buffer cache” to show why the “adaptive serial direct path read” feature is worth exploring in more detail.   




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