Dne 26.5.2011 16:39, Merlin Moncure napsal(a): > On Thu, May 26, 2011 at 9:01 AM, Tomas Vondra <tv@xxxxxxxx> wrote: >> Dne 26.5.2011 11:41, Alexander Farber napsal(a): >>> Also I wonder, how's shared memory used by PostgreSQL. >>> I'm irritated - how it could work with 32MB, >>> but now I've got suggestion to increase it >>> to 512MB (and it seems to work too...) >> >> Shared buffers are a 'database cache'. When the DB needs a block from a >> file (because that's where the data are stored), it reads the data into >> the cache. When the same block is needed again, it may be read from the >> cache (which is much faster). Unless there's not enough space to hold >> all the blocks - in that case the block may be removed from the cache >> and will be read from the disk again. > > *or the disk cache*. lowering shared buffers does not lower the > amount of ram in the system and thus does not lower the availability > of cache. If I may nitpick this point on your otherwise very > excellent email, this is exactly the type of thing that drives me > crazy about advice to raise shared buffers. It suggests you will get > less disk i/o which may or may not be the case (in fact, it can make > the i/o problem worse). If it does help i/o, it will probably not be > for the reasons you suspect. See my thread in -performance on this > topic. Yes, you're right. I didn't want to complicate the things further so I've skipped the part about page cache. Tomas -- Sent via pgsql-general mailing list (pgsql-general@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-general
