Message-ID: <43023BA0.5030608@arbash-meinel.com>
Date: Tue, 16 Aug 2005 14:16:48 -0500
From: John A Meinel <john@arbash-meinel.com>
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To: Anjan Dave <adave@vantage.com>
Cc: mudfoot@rawbw.com, pgsql-performance@postgresql.org
Subject: Re: choosing RAID level for xlogs
References: 
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In-Reply-To: 
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Anjan Dave wrote:
> Yes, that's true, though, I am a bit confused because the Clariion array
> document I am reading talks about how the write cache can eliminate the
> RAID5 Write Penalty for sequential and large IOs...resulting in better
> sequential write performance than RAID10.
>
> anjan
>

To give a shorter statement after my long one...
If you have enough cache that the controller can write out big chunks to
the disk at a time, you can get very good sequential RAID5 performance,
because the stripe size is large (so it can do a parallel write to all
disks).

But for small chunk writes, you suffer the penalty of the read before
write, and possible multi-disk read (depends on what is in cache).

RAID10 generally handles small writes better, and I would guess that
4disks would perform almost identically to 6disks, since you aren't
usually writing enough data to span multiple stripes.

If your battery-backed cache is big enough that you don't fill it, they
probably perform about the same (superfast) since the cache hides the
latency of the disks.

If you start filling up your cache, RAID5 probably can do better because
of the parallelization.

But small writes followed by an fsync do favor RAID10 over RAID5.

John
=:->

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