Re: Weird XFS WAL problem

Craig James wrote:
> This is really hard to believe, because the bonnie++ numbers and dd(1)
> numbers look good (see my original post).  But it's totally
> repeatable.  It must be some really unfortunate "just missed the next
> sector going by the write head" problem.

Commit performance is a separate number to measure that is not reflected
in any benchmark that tests sequential performance.  I consider it the
fourth axis of disk system performance (seq read, seq write, random
IOPS, commit rate), and directly measure it with the sysbench fsync test
I recommended already.  (You can do it with the right custom pgbench
script too).

You only get one commit per rotation on a drive, which is exactly what
you're seeing:  a bit under the 120 spins/second @ 7200 RPM.  Attempts
to time things just right to catch more than one sector per spin are
extremely difficult to accomplish, I spent a week on that once without
making any good progress.  You can easily get 100MB/s on reads and
writes but only manage 100 commits/second.

--
Greg Smith  2ndQuadrant US  Baltimore, MD
PostgreSQL Training, Services and Support
greg@2ndQuadrant.com   www.2ndQuadrant.us