On Wed, 2010-08-04 at 15:16 -0400, Greg Smith wrote: > Hannu Krosing wrote: > > There was ample space for keeping the indexes in linux cache (it has 1GB > > cached currently) though the system may have decided to start writing it > > to disk, so I suspect that most of the time was spent copying random > > index pages back and forth between shared buffers and disk cache. > > > > Low shared_buffers settings will result in the same pages more often > being written multiple times per checkpoint, Do you mean "written to disk", or written out from shared_buffers to disk cache ? > particularly index pages, > which is less efficient than keeping in the database cache and updating > them there. This is a slightly different issue than just the overhead > of copying them back and forth; by keeping them in cache, you actually > reduce writes to the OS cache. That's what I meant. Both writes to and read from the OS cache take a significant amount of time once you are not doing real disk I/O. > What I do to quantify that is...well, > the attached shows it better than I can describe; only works on 9.0 or > later as it depends on a feature I added for this purpose there. It > measures exactly how much buffer cache churn happened during a test, in > this case creating a pgbench database. > > -- > Greg Smith 2ndQuadrant US Baltimore, MD > PostgreSQL Training, Services and Support > greg@xxxxxxxxxxxxxxx www.2ndQuadrant.us > > -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
