From: NeilBrown <neilb@suse.de>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: John Robinson <john.robinson@anonymous.org.uk>,
David Brown <david.brown@hesbynett.no>,
Oliver Martin <oliver@volatilevoid.net>,
linux-raid@vger.kernel.org
Subject: Re: Creating a 3-disk RAID6 array
Date: Fri, 18 May 2012 17:57:00 +1000 [thread overview]
Message-ID: <20120518175700.61337765@notabene.brown> (raw)
In-Reply-To: <4FB5FAA7.4090401@zytor.com>
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On Fri, 18 May 2012 00:30:47 -0700 "H. Peter Anvin" <hpa@zytor.com> wrote:
> On 05/17/2012 07:04 AM, John Robinson wrote:
> > On 17/05/2012 14:18, David Brown wrote:
> > [...]
> >> Theoretically, a 3-disk RAID6 is like a 2-disk RAID5 or a 1-disk RAID1 -
> > [...]
> >> I can't think of any good reason for it /not/ to support 3-disk
> >> RAID6, as there is nothing in the algorithms to hinder it.
> >
> > I've a vague recollection of Peter Anvin saying the implementation is
> > optimised in such a way that it won't work.
> >
>
> Yes, it would have required introducing some odd special cases. A
> 3-disk RAID-6 is bitwise identical to a 3-disk RAID-1, so if Neil wants
> to he could add "instant reshaping" support in mdadm (add a disk to a
> 3-disk RAID-1 turning it into a 4-disk RAID-6; and similar for 2-disk
> RAID-1 into 3-disk RAID-5.)
>
> -hpa
>
Converting a 2-disk RAID-1 to a 3-disk RAID-5 is not instant.
You first convert a 2-disk RAID-1 to a 2-disk RAID-5. Then you re-stripe the
RAID-5 to have 3-disks. During this restripe, part of the array looks like a
2-disk RAID-5, and part looks like a 3-disk RAID-4.
To convert some sort of RAID1 to RAID6 we would need to be able to support a
3-disk RAID-6, if only for a relatively short time.
Possibly we could special case the parity-generation code to just copy the
data block when data-disks==1. That sound easy enough...
But the raid456 module needs to understand this
3-way-RAID1-pretending-to-be-RAID6, we cannot use the raid1 module to do it.
NeilBrown
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next prev parent reply other threads:[~2012-05-18 7:57 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-05-17 1:24 Creating a 3-disk RAID6 array Oliver Martin
2012-05-17 1:38 ` NeilBrown
2012-05-17 13:18 ` David Brown
2012-05-17 14:04 ` John Robinson
2012-05-18 7:24 ` David Brown
2012-05-18 7:30 ` H. Peter Anvin
2012-05-18 7:57 ` NeilBrown [this message]
2012-05-19 0:04 ` H. Peter Anvin
2012-05-17 21:02 ` Dan Williams
2012-05-17 23:27 ` Oliver Martin
2012-05-17 4:22 ` Bill Davidsen
2012-05-18 2:36 ` Stan Hoeppner
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