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@xxxxxxxxxxxxxxx www.2ndQuadrant.us -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
