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Subject: Re: Postgresql 9.0.6 Raid 5 or not please help.
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Am 23.12.2011 08:05, schrieb Scott Marlowe:
> On Thu, Dec 22, 2011 at 11:18 PM, tuanhoanganh<hatuan05@gmail.com>  wrote:
>> Thanks for your answer. But how performance between raid5 and one disk.
> One disk will usually win, 2 disks (in a mirror) will definitely win.
> RAID-5 has the highest overhead and the poorest performance,
> especially if it's degraded (1 drive out) that simple mirroring
> methods don't suffer from.  But even in an undegraded state it is
> usually the slowest method.  RAID-10 is generally the fastest with
> redundancy, and of course pure RAID-0 is fastest of all but has no
> redundancy.
>
> You should do some simple benchmarks with something like pgbench and
> various configs to see for yourself.  For extra bonus points, break a
> mirror (2 disk ->  1 disk) and compare it to RAID-5 (3 disk ->  2 disk
> degraded) for performance.  The change in performance for a RAID-1 to
> single disk degraded situation is usually reads are half as fast and
> writes are just as fast.  For RAID-5 expect to see it drop by a lot.
>
I'm not so confident that a RAID-1 will win over a single disk. When it 
comes to writes, the latency should be  ~50 higher (if both disk must 
sync), since the spindles are not running synchronously. This applies to 
softraid, not something like a battery-backend raid controller of course.

Or am I wrong here?