Re: wish for Linux MD mirrored raid types

["Keld Jørn Simonsen" <keld@keldix.com>]

On Mon, May 09, 2011 at 02:24:26PM +1000, NeilBrown wrote:
> 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.

Yes, I would like that we cocument the correspondance to the SNIA DDF
standard, and do other descriptions as you stipulate.

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

I see.

By "Nor is RAID-10." you mean RAID-1+0 or linux md raid10?

What is the difference between RAID-1E and Linux MD raid10?
Linux MD raid10 has "far" layout?
Linux MD raid10 can have more than 2 copies?
More stuff?



> > 
> > 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.

It is just that I have been around on other web pages for RAID, to
improve references to the linux-raid wiki and to correct errors in their
description of Linux RAID. Many - maybe most - people that write about
Linux RAID have a number of their details wrong.  That should be experts
conveying their expert wisdom to knowledge-hungry users. An example:
http://wiki.linuxquestions.org/wiki/RAID#RAID-10
They claim you need 4 disks for MD raid10. Another example is the German
wikipedia page on RAID - which was moderated and the moderator did not
accept my edits. This page does not describe Linux MD raid10. 

I think naming matters. If we could call Linux MD raid10 for raid1e
I think much would be achieved in terms of eliminating
misunderstandings.

Best regards
keld
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