Re: to enable O_DIRECT within postgresql

Greg: Thank you very much for your insightful comments on the performance of

direct io applied to postgres! That inspired me a lot.

Tom: thank you for the reference to man page!

On Fri, Jun 18, 2010 at 2:02 AM, Greg Smith <greg(at)2ndquadrant(dot)com> wrote:

> Daniel Ng wrote:
>
>> I am trying to enable the direct IO for the disk-resident
>> hash partitions of hashjoin in postgresql.
>>
>
> As Tom already mentioned this isn't working because of alignment issues.
> I'm not sure what you expect to achieve though. You should be warned that
> other than the WAL, every experiment I've ever seen that tries to add more
> direct I/O to the database has failed to improve anything; the result is
> neither barely noticeable, or a major performance drop. This is
> particularly futile if you're doing your research on Linux/ext3, where even
> if your code works delivers a speed up no one will trust it enough to ever
> merge and deploy it, due to the generally poor quality of that area of the
> kernel so far.
>
> This particular area is magnetic for drawing developer attention as it
> seems like there's a big win just under the surface if things were improved
> a bit. There isn't. On operating systems like Solaris where it's possible
> to prototype here by use mounting options to silently covert parts of the
> database to direct I/O, experiments in that area have all been
> disappointing. One of the presentations from Jignesh Shah at Sun covered
> his experiments in this area, can't seem to find it at the moment but I
> remember the results were not positive in any way.
>
> --
> Greg Smith 2ndQuadrant US Baltimore, MD
> PostgreSQL Training, Services and Support
> greg(at)2ndQuadrant(dot)com www.2ndQuadrant.us
>
>