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
