Re: Tuning PostgreSQL

On 29 Jul 2003, Ron Johnson wrote:

> On Tue, 2003-07-29 at 14:00, scott.marlowe wrote:
> > On 29 Jul 2003, Ron Johnson wrote:
> >
> > > On Tue, 2003-07-29 at 11:18, scott.marlowe wrote:
> > > > 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...
> > >
> > > Each with 14 devices? If so, isn't that a concentrated point of
> > > failure, even if the channels are 1/2 full?
> >
> > Yep. I've built one once before when BIG hard drives were 9 gigs. :-)
> >
> > And it is a point of concentrated failure, which brings me to my favorite
> > part about the LSI megaraid cards (which most / all perc3s are
> > apparently.)
> >
> > If you build a RAID1+0 or 0+1, you can seperate it out so each sub part is
> > on it's own card, and the other cards keep acting like one big card.
> > Assuming the bad card isn't killing your PCI bus or draining the 12V rail
> > or something.
>
> Sounds like my kinda card!
>
> Is the cache battery-backed up?

Yep

> How much cache can you stuff in them?

the old old old school MegaRAID428 could hold up to 128 Meg. I'm sure the
new ones can handle 512Meg or more.

> > > > > 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.
> > >
> > > Ok. You'd still saturate the 133MB/s PCI bus at 133/30 = 4.4 drives.
> >
> > But that's seq scan. For many database applications, random access
> > performance is much more important. Imagine 200 people entering
> > reservations of 8k or less each into a transaction processing engine.
> > Each transactions chance to hit an unoccupied spindle is what really
> > counts. If there's 30 spindles, each doing a stripe's worth of access all
> > the time, it's likely to never flood the channel.
> >
> > If random access is 1/4th the speed of seq scan, then you need to multiply
> > it by 4 to get the number of drives that'd saturate the PCI bus.
>
> Maybe it's just me, but I've never seen a purely TP system.

I think most of them are running under TPF on a mainframe in a basement
somewhere, like for airline reservations. I've never worked on one, but
met one of the guys who runs one, and they use 12 mainframes for 6 live
machines and each live machine has a failover machine behind it in sysplex
mode. I kept thinking of the giant dinosaurs in Jurassic park...

> Even if roll off the daily updates to a "reporting database" each
> night, some yahoo manager with enough juice to have his way still
> wants up-to-the-minute reports...

Just because it's TP doesn't mean it doesn't have real time reporting.
But expensive reports probably do get run at night.

> Better yet, the Access Jockey, who thinks s/he's an SQL whiz but
> couldn't JOIN himself out of a paper bag...

I've seen a few who got joins and unions and what not, but explaining fks
or transactions got me a glazed look... :-)