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 g...@2ndquadrant.com www.2ndQuadrant.us -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
