Re: to enable O_DIRECT within postgresql

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