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Re: A little RAID experiment

To: Stefan Ring <stefanrin@xxxxxxxxx>
Subject: Re: A little RAID experiment
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Thu, 11 Oct 2012 08:27:33 +1100
Cc: Linux fs XFS <xfs@xxxxxxxxxxx>
In-reply-to: <CAAxjCEwYyzTLTJk7SU0t=1SnT-V=7VadtKd0v9qj0soXtJ8XTg@xxxxxxxxxxxxxx>
References: <CAAxjCEzh3+doupD=LmgqSbCeYWzn9Ru-vE4T8tOJmoud+28FDQ@xxxxxxxxxxxxxx> <CAAxjCEwYyzTLTJk7SU0t=1SnT-V=7VadtKd0v9qj0soXtJ8XTg@xxxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Wed, Oct 10, 2012 at 04:57:47PM +0200, Stefan Ring wrote:
> Btw, one of our customers recently aquired new gear with HP SmartArray
> Gen8 controllers. Now they are something to get excited about! This is
> the kind of write performance I would expect from an expensive server
> product. Check this out (this is again my artificial benchmark as well
> as random write of 4K blocks):
> 
> SmartArray P400, 6 300G disks (10k, SAS) RAID 6, 256M BBWC:
                                                   ^^^^
.....

> SmartArray Gen8, 8 300G disks (15k, SAS) RAID 5, 2GB FBWC:
                                                   ^^^^

That's the reason for the difference in performance...

> So yeah, the disks are a bit faster. But what does that matter when
> there is such a huge difference otherwise?

Just inidicates that the working set for your test is much more
resident in the controller cache - has nothing to do with the disk
speeds. Tun a larger set of files/workload and the results will end
up a lot closer to disk speed instead of cache speed...

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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