On 2015-09-02 14:31:35 -0700, Josh Berkus wrote: > On 09/02/2015 02:25 PM, Tomas Vondra wrote: > > > > As I explained, spindles have very little to do with it - you need > > multiple I/O requests per device, to get the benefit. Sure, the DBAs > > should know how many spindles they have and should be able to determine > > optimal IO depth. But we actually say this in the docs: > > My experience with performance tuning is that values above 3 have no > real effect on how queries are executed.
I saw pretty much the opposite - the benefits seldomly were significant below 30 or so. Even on single disks. Which actually isn't that surprising - to be actually beneficial (that is, turn an IO into a CPU bound workload) the prefetched buffer needs to actually have been read in by the time its needed. In many queries processing a single heap page takes far shorter than prefetching the data from storage, even if it's on good SSDs. Therefore what you actually need is a queue of prefetches for the next XX buffers so that between starting a prefetch and actually needing the buffer ienough time has passed that the data is completely read in. And the point is that that's the case even for a single rotating disk! Greetings, Andres Freund -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
