From: "H. Peter Anvin" <hpa@zytor.com>
To: NeilBrown <neilb@suse.de>
Cc: Chris Pearson <kermit4@gmail.com>, linux-raid@vger.kernel.org
Subject: Re: with raid-6 any writes access all disks
Date: Thu, 27 Oct 2011 00:30:23 +0200 [thread overview]
Message-ID: <4EA889FF.90002@zytor.com> (raw)
In-Reply-To: <20111027082331.01e1fc7a@notabene.brown>
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On 10/26/2011 11:23 PM, NeilBrown wrote:
> On Wed, 26 Oct 2011 16:01:19 -0500 Chris Pearson
> <kermit4@gmail.com> wrote:
>
>> In 2.6.39.1, any writes to a raid-6 array cause all disks to be
>> accessed. Though I don't understand the math behind raid-6, I
>> have tested on LSI cards that it is possible to only access 3
>> disks.
>
> You are correct. md/raid6 doesn't do the required maths.
>
> i.e. it always adds all data together to calculate the parity. It
> never subtracts old data from the parity, then add new data.
>
> This was a decision made by the original implementer (hpa) and
> no-one has offered code to change it.
>
> (yes, I review and accept patches :-)
>
This was based on benchmarks at the time that indicated that
performance suffered more than it helped. However, since then CPUs
have gotten much faster whereas disks haven't. There was also the
issue of getting something working reliably first.
Getting a set of hardware acceleration routines for arbitrary GF
multiplies (as is possible with SSSE3) might change that tradeoff
dramatically.
-hpa
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next prev parent reply other threads:[~2011-10-26 22:30 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-10-26 21:01 with raid-6 any writes access all disks Chris Pearson
2011-10-26 21:23 ` Peter W. Morreale
2011-10-26 21:23 ` NeilBrown
2011-10-26 22:30 ` H. Peter Anvin [this message]
2011-10-27 9:29 ` David Brown
2011-10-27 12:22 ` H. Peter Anvin
2011-10-27 13:05 ` David Brown
2011-11-01 22:22 ` H. Peter Anvin
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