On Tue, Apr 26, 2011 at 7:30 AM, Robert Haas <robertmhaas@xxxxxxxxx> wrote: > On Apr 14, 2011, at 2:49 AM, Claudio Freire <klaussfreire@xxxxxxxxx> wrote: >> This particular factor is not about an abstract and opaque "Workload" >> the server can't know about. It's about cache hit rate, and the server >> can indeed measure that. > > The server can and does measure hit rates for the PG buffer pool, but to my knowledge there is no clear-cut way for PG to know whether read() is satisfied from the OS cache or a drive cache or the platter. Isn't latency an indicator? If you plot latencies, you should see three markedly obvious clusters: OS cache (microseconds), Drive cache (slightly slower), platter (tail). I think I had seen a study of sorts somewhere[0]... Ok, that link is about sequential/random access, but I distinctively remember one about caches and CAV... [0] http://blogs.sun.com/brendan/entry/heat_map_analytics -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
