From: NeilBrown <neilb@suse.de>
To: Chris Pearson <kermit4@gmail.com>
Cc: linux-raid@vger.kernel.org, "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: with raid-6 any writes access all disks
Date: Thu, 27 Oct 2011 08:23:31 +1100 [thread overview]
Message-ID: <20111027082331.01e1fc7a@notabene.brown> (raw)
In-Reply-To: <CAGtzr3c65BXDfWZq4ejKwCZv7hPZmdCpvQP17RgFS-Vrn2YF5Q@mail.gmail.com>
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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 :-)
NeilBrown
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next prev parent reply other threads:[~2011-10-26 21:23 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 [this message]
2011-10-26 22:30 ` H. Peter Anvin
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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