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In-Reply-To: <D89BE95F-60D2-431B-A82D-53378392AAC3@autouncle.com>
References: <D89BE95F-60D2-431B-A82D-53378392AAC3@autouncle.com>
Date: Mon, 17 Feb 2014 16:40:21 +0100
Subject: Re: Optimal settings for RAID controller - optimized for
 writes
From: "Tomas Vondra" <tv@fuzzy.cz>
To: =?utf-8?B?Ik5pZWxzIEtyaXN0aWFuIFNjaGrDuGR0Ig==?=
 <nielskristian@autouncle.com>
Cc: "pgsql-performance@postgresql.org list" <pgsql-performance@postgresql.org>
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On 17 Únor 2014, 16:03, Niels Kristian Schjødt wrote:
> Hi,
>
> I’m kind of a noob when it comes to setting up RAID controllers and
> tweaking them so I need some advice here.
>
> I’m just about to setup my newly rented DELL R720 12. gen server. It’s
> running a single Intel Xeon E5-2620 v.2 processor and 64GB ECC ram. I have
> installed 8 300GB SSDs in it. It has an PERC H710 raid controller (Based
> on the LSI SAS 2208 dual core ROC).
>
> Now my database should be optimized for writing. UPDATEs are by far my
> biggest bottleneck.

I think it's pretty difficult to answer this without a clear idea of how
much data the UPDATEs modify etc. Locating the data may require a lot of
reads too.

> Firstly: Should I just put all 8 drives in one single RAID 10 array, or
> would it be better to have the 6 of them in one RAID 10 array, and then
> the remaining two in a separate RAID 1 array e.g. for having WAL log dir
> on it’s own drives?

This used to be done to separate WAL and data files onto separate disks,
as the workloads are very different (WAL is almost entirely sequential
writes, access to data files is often random). With spinning drives this
made a huge difference as the WAL drives were doing just seq writes, but
with SSDs it's not that important anymore.

If you can do some testing, do it. I'd probably create a RAID10 on all 8
disks.

> Secondly: Now what settings should I pay attention to when setting this
> up, if I want it to have optimal write performance (cache behavior, write
> back etc.)?

I'm wondering whether the controller (H710) actually handles TRIM well or
not. I know a lot of hardware controllers tend not to pass TRIM to the
drives, which results in poor write performance after some time, but I've
been unable to google anything about TRIM on H710.

Tomas


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