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 <g...@2ndquadrant.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 > g...@2ndquadrant.com www.2ndQuadrant.us > >
