Re: wish for Linux MD mirrored raid types

From: NeilBrown <neilb@suse.de>
To: "Keld Jørn Simonsen" <keld@keldix.com>
Cc: Miles Fidelman <mfidelman@meetinghouse.net>, linux-raid@vger.kernel.org
Subject: Re: wish for Linux MD mirrored raid types
Date: Mon, 9 May 2011 14:24:26 +1000	[thread overview]
Message-ID: <20110509142426.564aae2c@notabene.brown> (raw)
In-Reply-To: <20110509034036.GA17111@www2.open-std.org>

On Mon, 9 May 2011 05:40:36 +0200 Keld Jørn Simonsen <keld@keldix.com> wrote:

> On Sun, May 08, 2011 at 05:25:24PM -0400, Miles Fidelman wrote:
> > Luca Berra wrote:
> > >On Fri, May 06, 2011 at 09:17:52AM +0200, Keld Jørn Simonsen wrote:
> > >>I would like linux MD raid10 functionality to be part of the Linux MD
> > >>RAID1 module, and be called raid1. This is in accordance with the
> > >then we need rename the current raid1 functionality to 'mirror'. in
> > >order to avoid further confusion.
> > >besides, current raid10 does not support resizing, so the feature
> > >should be added before ditching 'mirror'
> > >
> > 
> > The current md RAID1 does exactly what RAID1 is supposed to do.
> 
> I changed my mind a little.
> 
> I think we should follow SNIA wrt. RAID1 - all that SNIA would say is
> RAID1 we also should be able to do as RAID1 - that would include
> raid10-offset which directly was implemented in the Linux kernel
> because of the SNIA RAID1 specification. It should also include raid10-far
> in so far it is a raid1 type - say a raid10,f2 with only 2 disks. 

I think you misread SNIA-DDF.

In DDF, arrays with a PRL (Primary RAID Level) of 1 can have an RLQ (RAID
level qualifier) of 0 or 1.
RLQ = 0  ->  RAID1 with 2 devices
RLQ = 1  ->  RAID1 with 3 devices.

DDF also devices a PRL of '11' which it calls "RAID-1E" (though this term
only appears once in the DDFv1.2 spec)

For PRL = 11  there are two options

RLQ = 0 -> Integrated Adjacent Stripe Mirroring
RLQ = 1 -> Integrated Offset Stripe Mirroring.

These correspond to md/raid10 "near2" and "offset2".

So DDF: RAID-1E  corresponds to md: RAID-10

So an 'E' rather than a '0'.

I would not be against allowing mdadm to accept "raid1e" as a synonym for
'raid10', and mentioning the alternate name in the documentation would be
entirely appropriate.

But RAID-1E is not RAID-1.  Nor is RAID-10.

> 
> Then we should keep the raid10 stuff.
> 
> > The md RAID10 is a very specific, and unique approach that has 
> > similarities to, but is distinct from, RAID1+0, RAID0+1, RAID5, and RAID6.
> 
> Yes, Linux MD raid10 is a very distinct type. We should talk with SNIA to get it
> recognized.
> 
> > What say we leave the names alone.  Just beause one person is confused is 
> > no reason to further confuse things.
> 
> The confusion is not just one person. The confusion is unbelievable common, 
> and has proven to be very hard to eliminate. If we align with the SNIA
> standard, and further get the standard to align with us, then we should
> have a chance in say 5 years to have reduced the confusion considerably.

The world is full of confusion that is hard to eliminate.

The problem here I think is simply people who do not educate themselves,
either because they cannot be bothered, or because they cannot easily find
the materials.

The first is not really a fixable problem.
The second we can address.  Improve the already-good wiki or add more text to
the man pages.  Have an aim that every general-information question can be
answered by simply posting a like or a passage from the man page.

That would be really worthwhile.

Changing names around is, I think, less valuable.

Thanks,
NeilBrown

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