On 03/12/2013 05:49 PM, Gregg Jaskiewicz wrote:
> So out of 6 disks then having 4 in Raid 1+0 configuration and other
> two in mirror for WAL. That's another option then for me to test.
That is an option, but it's not necessarily a good one. If all you have
are six disks, you are probably better off just doing a big RAID-10 for
everything.
As Alban mentioned, you would do best to stay away from RAID-5 as that
will basically obliterate your write speeds, which is especially
critical for WAL files.
There is one caveat to all of this, of course. What you plan to use the
server for has a critical impact on these decisions.
What it boils down to, is that six spindles is not very many. You don't
really have enough to dedicate any to your WAL files without drastically
cutting the available IOPS for your regular data. In addition, with any
of these setups, you'll want a hot spare in case of drive failure. If
you have six disks available, you really only have five. That's not an
even number, so one is wasted in RAID-10, and you may find in your
RAID-5 tests that it doesn't work well for long-term use.
The truth is that you can get by for quite a while on this no matter how
you split up those six spindles. Until you need a lot of heavy disk
reads or writes. Then it's going to hurt. So ask yourself what kind of
load or usage you expect, and plan accordingly.
I have to tell you though, we had a server with twelve spindles three
years ago, and it barely kept up with our transaction load. We had two
hot spares, a RAID-1, and 8-disks in a RAID-10. Several pgbench tests
back then showed that our RAID-10 could only adequately serve 1800TPS
directly, and we needed at least 6000. Ultimately, it lead to us
switching to NVRAM (SSD) for high TPS data, and creating a tablespace on
a RAID-10 for archived or low-priority data.
But we got by on those original 12 spindles for a couple years. If your
data needs are less, you can probably do OK with six. For a while,
anyway. :)
--
Shaun Thomas
OptionsHouse | 141 W. Jackson Blvd. | Suite 500 | Chicago IL, 60604
312-676-8870
sthomas@optionshouse.com
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