Re: Tuning PostgreSQL

On 29 Jul 2003, Ron Johnson wrote:

> On Tue, 2003-07-29 at 10:14, Vivek Khera wrote:
> > >>>>> "GS" == Greg Stark <gsstark(at)mit(dot)edu> writes:
> >
> > GS> "scott.marlowe" <scott(dot)marlowe(at)ihs(dot)com> writes:
> >
> > GS> But you have to actually test your setup in practice to see if it
> > GS> hurts. A big data warehousing system will be faster under RAID5
> > GS> than under RAID1+0 because of the extra disks in the
> > GS> stripeset. The more disks in the stripeset the more bandwidth you
> > GS> get.
> >
> > Anyone have ideas on 14 spindles? I just ordered a disk subsystem
> > with 14 high speed (U320 15kRPM) SCSI disks to hook up with a dell
> > PERC3/DC controller (only 128MB cache, though).
>
> 14 drives on one SCSI card, eh? I'd be worried about saturating
> the bus.

I'm pretty sure those PERCs are based on the megaraid cards, which can
handle 3 or 4 channels each...

> Maybe it's an old rule of thumb, but I would fill a SCSI chain
> more than half full.

It's an old rule of thumb, but it still applies, it just takes more drives
to saturate the channel. Figure ~ 30 to 50 MBytes a second per drive, on
a U320 port it would take 10 drives to saturate it, and considering random
accesses will be much slower than the max ~30 megs a second off the
platter rate, it might take more than the max 14 drives to saturate U320.