On Tue, 2006-08-08 at 17:53, Thomas F. O'Connell wrote:
> On Aug 8, 2006, at 5:28 PM, Joshua D. Drake wrote:
>
> > Thomas F. O'Connell wrote:
> >> On Aug 8, 2006, at 4:49 PM, Joshua D. Drake wrote:
> >>>> I am considering a setup such as this:
> >>>> - At least dual cpu (possibly with 2 cores each)
> >>>> - 4GB of RAM
> >>>> - 2 disk RAID 1 array for root disk
> >>>> - 4 disk RAID 1+0 array for PGDATA
> >>>> - 2 disk RAID 1 array for pg_xlog
> >>>> Does anyone know a vendor that might be able provide such setup?
> >> Wouldn't it be preferable to put WAL on a multi-disk RAID 10 if
> >> you had the opportunity? This gives you the redundancy of RAID 1
> >> but approaches the performance of RAID 0, especially as you add
> >> disks to the array. In benchmarking, I've seen consistent success
> >> with this approach.
> >
> > WALL is written in order so RAID 1 is usually fine. We also don't
> > need journaling for WAL so the speed is even faster.
>
> In which case, which is theoretically better (since I don't have a
> convenient test bed at the moment) for WAL in a write-heavy
> environment? More disks in a RAID 10 (which should theoretically
> improve write throughput in general, to a point) or a 2-disk RAID 1?
> Does it become a price/performance question, or is there virtually no
> benefit to throwing more disks at RAID 10 for WAL if you turn off
> journaling on the filesystem?
Actually, the BIGGEST win comes when you've got battery backed cache on
your RAID controller. In fact, I'd spend money on a separate RAID
controller for xlog with its own cache hitting a simple mirror set
before I'd spring for more drives on pg_xlog. The battery backed cache
on the pg_xlog likely wouldn't need to be big, just there and set to
write-back.
Then put all the rest of your cash into disks on a big RAID 10 config,
and as big of a battery backed cache as you can afford for it and memory
for the machine.
