On Wed, 17 Sep 2003, Tom Lane wrote: > Michael Adler <[EMAIL PROTECTED]> writes: > > I have been experimenting with a new Seagate Cheetah 10k-RPM SCSI to > > compare with a cheaper Seagate Barracuda 7200-RPM IDE (each in a > > single-drive configuration). The Cheetah definately dominates the generic > > IO tests such as bonnie++, but fares poorly with pgbench (and other > > postgresql operations). > > It's fairly common for ATA drives to be configured to lie about write > completion (ie, claim write-complete as soon as data is accepted into > their onboard RAM buffer), whereas SCSI drives usually report write > complete only when the data is actually down to disk. The performance > differential may thus be coming at the expense of reliability. If you > run Postgres with fsync off, does the differential go away?
Yes, they both perform equally at about 190 tps with fsync off. The culprit turns out to be write-caching on the IDE drive. It is enabled by default, but can be disabled with "hdparm -W0 /dev/hdx". After it is disabled, the tps are proportional to rpms. There's an (2001) Linux thread on this if anyone is interested: http://www.ussg.iu.edu/hypermail/linux/kernel/0103.0/0331.html So the quesiton is whether it is ever sensible to use write-caching and expect comparable persistence. Thanks, Michael Adler ---------------------------(end of broadcast)--------------------------- TIP 8: explain analyze is your friend