See below:
On Wed, Mar 10, 2010 at 9:28 PM, Paul McGarry <p...@paulmcgarry.com> wrote: > Hi there, > > I'm after a little bit of advice on the shared_buffers setting (I have > read the various docs on/linked from the performance tuning wiki page, > some very helpful stuff there so thanks to those people). > > I am setting up a 64bit Linux server running Postgresql 8.3, the > server has 64gigs of memory and Postgres is the only major application > running on it. (This server is to go alongside some existing 8.3 > servers, we will look at 8.4/9 migration later) > > I'm basically wondering how the postgresql cache (ie shared_buffers) > and the OS page_cache interact. The general advice seems to be to > assign 1/4 of RAM to shared buffers. > > I don't have a good knowledge of the internals but I'm wondering if > this will effectively mean that roughly the same amount of RAM being > used for the OS page cache will be used for redundantly caching > something the Postgres is caching as well? > > IE when Postgres reads something from disk it will go into both the OS > page cache and the Postgresql shared_buffers and the OS page cache > copy is unlikely to be useful for anything. > > If that is the case what are the downsides to having less overlap > between the caches, IE heavily favouring one or the other, such as > allocating shared_buffers to a much larger percentage (such as 90-95% > of expected 'free' memory). > > Pg apparently does not have an option of using direct IO with reads which some other databases do (the O_DIRECT mode). Therefore, double-buffering with read operations seems unavoidable. Counterintuitively, it may be a good idea to just rely on OS buffering and keep shared_buffers rather small, say, 512MB. VJ > Paul > > -- > Sent via pgsql-performance mailing list (pgsql-performance@postgresql.org) > To make changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-performance >
