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	Mon, 02 May 2005 07:38:19 PDT
Date: Mon, 2 May 2005 07:38:19 -0700 (PDT)
From: Arne "Wörner" <arne_woerner@yahoo.com>
To: Allen <bsdlists@rfnj.org>
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cc: freebsd-performance@freebsd.org
Subject: Re: Very low disk performance on 5.x
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--- Allen <bsdlists@rfnj.org> wrote:
> Scenario B, verified read enabled:
> 1. RAID card reads up ALL blocks in the stripe (5 reads).
> 2. RAID card pretends the block requested is on a "degraded"
> drive, and 
> calculates it from the other 3 + the XOR stripe.
> 3. RAID card reports the value back, or tosses some kind of
> error.
> 
> You can see, the cache just doesn't play a part in what I was
> describing, 
> which is basically the array performing as though it is degraded
> when in 
> fact it is not, to catch failures that would otherwise be
> missed.
> 
That would be a funny implementation. Step one could be done
mostly from cache in case of sequential reads from a device (like
/dev/ad0s1f or so). In this thread we always looked at sequential
reads, as far as I recall...

-Arne


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