Stephen Tyler wrote:
The times are apparently in milliseconds. But that doesn't make any
sense for the 7200rpm SATA HDD, given it has an 8.3ms rotation time.
And the numbers for the SSD are basically identical to the HDD, apart
from the bare write timing.
Do I conclude that the sync timings are all faked by the OS?
PostgreSQL on OS X uses a special fsync method by default named
'fsync_writethrough'. If you do this on your database you should be
able to confirm it's using it:
show wal_sync_method;
This method includes a call to Apple's API to make sure the cache is
properly flushed on the drives. Lots more details about how all this
works at http://www.westnet.com/~gsmith/content/postgresql/TuningPGWAL.htm
I'm not sure if the test_fsync tool actually tests this particular
wal_sync_method though, so on OS X you may only be getting reports that
don't include a proper drive cache flush. The server itself does the
right thing. Making test_fsync do exactly the same thing as the real
WAL write in all cases has been a lingering TODO item for a while now, I
submitted a patch to improve the whole thing a while ago but it's not
really robust in all situations yet. Unfortunately I think you're
getting bad data from the test tool, and I'm not sure of a source for a
similar one that includes the right drive flushing behavior for OS X.
(As a general note, there's a similar call on Windows, and I believe it
also suffers from this problem when running test_fsync ; that little
test utility has really only been tested well on UNIX-ish systems).
What you can do in order to test fsync rate as a side-effect of other
work the server is to run pgbench with a custom script. If you grab a
copy of
http://git.postgresql.org/gitweb?p=pgbench-tools.git;a=blob_plain;f=tests/insert.sql;hb=945164aaadf50ea645750fe77c4ffcecafb5c353and
save that as "insert.sql", then do:
createdb pgbench
pgbench -i -s 10 pgbench
pgbench -s 10 -t 1000 -c 1 -f insert.sql
This will tell you how many insert statements/second are committing,
which is basically just a tiny write to the WAL followed by an fsync.
That's basically the fsync rate of the system, as long as you don't get
a checkpoint in the middle slowing it down.
--
Greg Smith 2ndQuadrant Baltimore, MD
PostgreSQL Training, Services and Support
greg@xxxxxxxxxxxxxxx www.2ndQuadrant.com
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