The conclusion I read was that Linux O_SYNC behaves like O_DSYNC on other systems. For WAL, this seems satisfactory?
Personally, I use fdatasync(). I wasn't able to measure a reliable difference for my far more smaller databases, and fdatasync() seems reliable and fast enough, that fighting with O_SYNC doesn't seem to be worth it. Also, technically speaking, fdatasync() appeals more to me, as it allows the system to buffer while it can, and the application to instruct it across what boundaries it should not buffer. O_SYNC / O_DSYNC seem to imply a requirement that it does a synch on every block. My gut tells me that fdatasync() gives the operating system more opportunities to optimize (whether it does or not is a different issue :-) ).
Cheers,
mark
On 06/17/2010 11:29 PM, Greg Smith wrote:
Mark Kirkwood wrote: Now I recall some discussion about this enabling direct io and the general flakiness of this on Linux, so is the option regarded as safe?
No one has ever refuted the claims in http://archives.postgresql.org/pgsql-hackers/2007-10/msg01310.php that it can be unsafe under a heavy enough level of mixed load on RHEL5. Given the performance benefits are marginal on ext3, I haven't ever considered it worth the risk. (I've seen much larger gains on Linux+Veritas VxFS). From what I've seen, recent Linux kernel work has reinforced that the old O_SYNC implementation was full of bugs now that more work is being done to improve that area. My suspicion (based on no particular data, just what I've seen it tested with) is that it only really worked before in the very specific way that Oracle does O_SYNC writes, which is different from what PostgreSQL does.
P.S. Be wary of expecting pgbench to give you useful numbers on a single run. For the default write-heavy test, I recommend three runs of 10 minutes each (-T 600 on recent PostgreSQL versions) before I trust any results it gives. You can get useful data from the select-only test in only a few seconds, but not the one that writes a bunch.
--
Greg Smith 2ndQuadrant US Baltimore, MD
PostgreSQL Training, Services and Support
greg@2ndQuadrant.com www.2ndQuadrant.us
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Mark Mielke <mark@mielke.cc>
