On Sat, Mar 1, 2008 at 3:53 PM, Craig James <[EMAIL PROTECTED]> wrote:
> Joshua D. Drake wrote:
> > On Sat, 01 Mar 2008 10:06:54 -0800
> > Craig James <[EMAIL PROTECTED]> wrote:
> >
> >> We're upgrading to a medium-sized server, a Dell PowerEdge 2950,
> >> dual-quad CPU's and 8 GB memory. This box can hold at most 8 disks
> >> (10K SCSI 2.5" 146 GB drives) and has Dell's Perc 6/i RAID controller.
> >>
> >> I'm thinking of this:
> >>
> >> 6 disks RAID 1+0 Postgres data
> >> 1 disk WAL
> >> 1 disk Linux
> >>
> >> I've often seen RAID 1 recommended for the WAL. Is that strictly for
> >> reliability, or is there a performance advantage to RAID 1 for the
> >> WAL?
> >>
> >> It seems to me separating the OS and WAL on two disks is better than
> >> making a single RAID 1 and sharing it, from a performance point of
> >> view.
> >
> > This scares me... You lose WAL you are a goner. Combine your OS and
> > WAL into a RAID 1.
>
> Right, I do understand that, but reliability is not a top priority in this
> system. The database will be replicated, and can be reproduced from the raw
> data. It's not an accounting system, it finds scientific results. That's
> not to say I *won't* take your advice, we may in fact combine the OS and WAL
> on one disk. Reliability is a good thing, but I need to know all of the
> tradeoffs, so that I can weigh performance, reliability, and cost and make
> the right choice.
In that case you could always make the data partition a 6 disk RAID-0.
> So my question still stands: From a strictly performance point of view,
> would it be better to separate the OS and the WAL onto two disks? Is there
> any performance advantage to RAID 1? My understanding is that RAID 1 can
> give 2x seek performance during read, but no advantage during write. For the
> WAL, it seems to me that RAID 1 has no performance benefits, so separating
> the WAL and OS seems like a peformance advantage.
Yes, Only on Reads. Correct.
> Another option would be:
>
>
> 4 disks RAID 1+0 Postgres data
> 2 disks RAID 1 WAL
> 1 disk Linux
> 1 disk spare
>
> This would give us reliability, but I think the performance would be
> considerably worse, since the primary Postgres data would come from 4 disks
> instead of six.
Performance-wise, RAID-10 with n disks is about the same as RAID-0
with n/2 disks. So, you're losing abot 1/3 of your peak performance,
assuming 100% efficient controllers and you aren't bottlenecking I/O
with > 4 disks.
> I guess we could also consider:
>
>
> 4 disks RAID 1+0 Postgres data
> 4 disks RAID 1+0 WAL and Linux
>
> Or even
>
> 8 disks RAID 1+0 Everything
It really depends on the controller. Battery backed write cache?
Then the one big everything is often faster than any other method. No
BB cache? Then splitting them up will help.
> I suppose the thing to do is get the system, and run bonnie on various
> configurations. I've never run bonnie before -- can I get some useful
> results without a huge learning curve?
Yes, it's fairly easy to drive. It'll tell you more about your
controller than anything else, which is very useful information. The
way a different controllers behaves with different configurations can
be very very different from one controller to the next.
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