Re: Tuning Tips for a new Server

On Aug 17, 2011, at 2:14 PM, Scott Marlowe wrote:

> On Wed, Aug 17, 2011 at 12:56 PM, Tomas Vondra <tv(at)fuzzy(dot)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.

I suppose this is my confusion. Or rather I am curious about this. On my current production database the pg_xlog directory is 8Gb (our total database is 200Gb). Does this warrant a totally separate setup (and hardware) than PGDATA?