On Sun, Sep 27, 2009 at 11:18 AM, Sam Mason <sam@samason.me.uk> wrote:
> On Sun, Sep 27, 2009 at 06:05:51PM +0200, Gerhard Wiesinger wrote:
>> A google research has shown that Gregory Stark already worked on that issue
>> (see references below) but as far as I saw only on bitmap heap scans.
>
> Greg Stark's patches are about giving the IO subsystem enough
> information about where the random accesses will be ending up next.
> This is important, but almost completely independent from the case
> where you know you're doing sequential IO, which is what you seem to be
> talking about.
FWIW I did work to write code to use FADV_SEQUENTIAL and FADV_RANDOM
but couldn't demonstrate any performance improvement. Basically
Postgres was already capable of saturating any raid controller I could
test doing a normal sequential scan with 8k block sizes and no special
read-ahead advice.
--
greg