On Wed, Aug 17, 2011 at 12:56 PM, Tomas Vondra <tv@fuzzy.cz> wrote:
>
> I think you've mentioned the database is on 6 drives, while the other
> volume is on 2 drives, right? That makes the OS drive about 3x slower
> (just a rough estimate). But if the database drive is used heavily, it
> might help to move the xlog directory to the OS disk. See how is the db
> volume utilized and if it's fully utilized, try to move the xlog
> directory.
>
> The only way to find out is to actualy try it with your workload.
This is a very important point. I've found on most machines with
hardware caching RAID and 8 or fewer 15k SCSI drives it's just as
fast to put it all on one big RAID-10 and if necessary partition it to
put the pg_xlog on its own file system. After that depending on the
workload you might need a LOT of drives in the pg_xlog dir or just a
pair. Under normal ops many dbs will use only a tiny % of a
dedicated pg_xlog. Then something like a site indexer starts to run,
and writing heavily to the db, and the usage shoots to 100% and it's
the bottleneck.
