Re: How to setup disk spindles for best performance

Hi Scott,

Great info! Our RAID card is at the moment a ICP vortex (Adaptec)
ICP5165BR, and I'll be using it with Ubuntu server 8.04. I tried
OpenSolaris, but it yielded even more terrible performance, specially
using ZFS.. I guess that was just a missmatch. Anyway, I'm going to
return the controller, because it does not scale very well with more
that 4 disks in raid 10. Bandwidth is limited to 350MB/sec, and IOPS
scale badly with extra disks...

So I guess, I'll be waiting for another controller first. The idea for
xlog + os on 4 disk raid 10 and the rest for the data sound good :) I
hope it will turn out that way too.. First another controller..

Regards,

Christiaan

Scott Carey wrote:
> Indexes will be random write workload, but these won't by synchronous
> writes and will be buffered by the raid controller's cache. Assuming
> you're using a hardware raid controller that is, and one that doesn't
> have major performance problems on your platform. Which brings those
> questions up --- what is your RAID card and OS?
>
> For reads, if your shared_buffers is large enough, your heavily used
> indexes won't likely go to disk much at all.
>
> A good raid controller will typically help distribute the workload
> effectively on a large array.
>
> You probably want a simple 2 disk mirror or 4 disks in raid 10 for
> your OS + xlog, and the rest for data + indexes -- with hot spares IF
> your card supports them.
>
> The biggest risk to splitting up data and indexes is that you don't
> know how much I/O each needs relative to each other, and if this isn't
> a relatively constant ratio you will have one subset busy while the
> other subset is idle.
> Unless you have extensively profiled your disk activity into index and
> data subsets and know roughly what the optimal ratio is, its probably
> going to cause more problems than it fixes.
> Furthermore, if this ratio changes at all, its a maintenance
> nightmare. How much each would need in a perfect world is application
> dependant, so there can be no general recommendation other than:
> don't do it.
>
> On Thu, Aug 21, 2008 at 1:34 AM, Christiaan Willemsen
> <cwillemsen(at)technocon(dot)com <mailto:cwillemsen(at)technocon(dot)com>> wrote:
>
> Thanks Joshua,
>
> So what about putting the indexes on a separate array? Since we do
> a lot of inserts indexes are going to be worked on a lot of the time.
>
> Regards,
>
> Christiaan
>
>
> Joshua D. Drake wrote:
>
> Christiaan Willemsen wrote:
>
> So, what you are basically saying, is that a single mirror
> is in general more than enough to facilitate the
> transaction log.
>
>
> http://www.commandprompt.com/blogs/joshua_drake/2008/04/is_that_performance_i_smell_ext2_vs_ext3_on_50_spindles_testing_for_postgresql/
>
> http://wiki.postgresql.org/wiki/HP_ProLiant_DL380_G5_Tuning_Guide
>
> And to answer your question, yes. Transaction logs are written
> sequentially. You do not need a journaled file system and raid
> 1 is plenty for most if not all work loads.
>
> Sincerely,
>
> Joshua D. Drake
>
>
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