From mboxrd@z Thu Jan  1 00:00:00 1970
From: NeilBrown <neilb@suse.de>
Subject: Re: with raid-6 any writes access all disks
Date: Thu, 27 Oct 2011 08:23:31 +1100
Message-ID: <20111027082331.01e1fc7a@notabene.brown>
References: <CAGtzr3c65BXDfWZq4ejKwCZv7hPZmdCpvQP17RgFS-Vrn2YF5Q@mail.gmail.com>
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To: Chris Pearson <kermit4@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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