Message-ID: <4DFA4B89.6080504@krogh.cc>
Date: Thu, 16 Jun 2011 20:29:29 +0200
From: Jesper Krogh <jesper@krogh.cc>
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To: Haestan <haestan@gmail.com>
CC: pgsql-performance@postgresql.org
Subject: Re: Performance advice for a new low(er)-power server
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On 2011-06-16 17:09, Haestan wrote:
> I am evaluating hardware for a new PostgreSQL server. For reasons
> concerning power consumption and available space it should not have
> more than 4 disks (in a 1U case), if possible. Now, I am not sure what
> disks to use and how to layout them to get the best performance.
What is your data:memory-size ratio? Can you afford to have everything
in memory and only have the disks to be able to sustain writes?

> The cheaper option would be to buy 15k Seagate SAS disks with a 3ware
> 9750SA (battery backed) controller. Does it matter whether to use a
> 4-disk RAID10 or 2x 2-disk RAID1 (system+pg_xlog , pg_data) setup? Am
> I right that both would be faster than just using a single 2-disk
> RAID1 for everything?
>
> A higher end option would be to use 2x 64G Intel X-25E SSD's with a
> LSI MegaRAID 9261 controller for pg_data and/or pg_xlog and 2x SAS
> disks for the rest. Unfortunately, these SSD are the only ones offered
> by our supplier and they don't use a supercapacitor, AFAIK. Therefore
> I would have to disable the write cache on the SSD's somehow and just
> use the cache on the controller only. Does anyone know if this will
> work or even uses such a setup.
Any SSD is orders of magnitude better than any rotating drive
in terms of random reads. If you will benefit depends on your
data:memory ratio..

> Furthermore, the LSI MegaRAID 9261 offers CacheCade which uses SSD
> disks a as secondary tier of cache for the SAS disks. Would this
> feature make sense for a PostgreSQL server, performance wise?
I have one CacheCade setup...  not a huge benefit but it seems
measurable. (but really hard to test).  .. compared to a full
SSD-setup I wouldn't consider it at all.

-- 
Jesper