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@xxxxxxxxxxxxxxx www.2ndQuadrant.us
--
Mark Mielke <mark@xxxxxxxxx>
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