linux-kernel - Re: RAID60

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Date:	Thu, 23 Jul 2009 09:06:07 +1000 (EST)
From:	"NeilBrown" <neilb@...e.de>
To:	"mark delfman" <markdelfman@...glemail.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: RAID60 - MD / LVM

On Wed, July 22, 2009 7:04 pm, mark delfman wrote:
> Hi All,
>
> Firstly let me apologise for what maybe a simple question, I have
> tried to google and search this out.
>
> The question is simply the best way to create a RAID60 or any nested
> RAID.  Looking at the examples it seems the approach is to create 2 x
> RAID6s and then simply RAID0 on top.  However, is there any advantage
> in using MD to create the RAID0 or would you see the same overall
> result by using LVM to stripe (R0) the 2 x RAID6s?

Most of the difference between using MD and LVM to do the striping would
be simply in the fact that different tools are used, so the best advice
would probably be to use whatever you are more familiar with.
In 2.6.31, md/raid0 will support a chunk size that is not a power of
2.  I don't think LVM supports that (yet).
That was added specifically for this sort of set up, so that the
RAID0 chuck can align with the RAID6 stripe.  Exactly how important
that alignment is probably depends on your workload.  I haven't really
thought it though enough to be able to give any guidance.

>
> Given that I see a lot of posts RE: RAID10, then I suspect that the
> MD RAID0 code has some understanding of or tuning for nested RAIDs,
> which I guess LVM may not have.

MD has a separate RAID10 module which stripes multiple copies of the
data across multiple drive.  It is simpler and more flexible than
RAID0 over RAID1.  There is no similar RAID60 module as there is a lot
less room for gains in flexibility.

>
> It would seem the answer is obvious as it makes sense to simply use
> MD, but we have an internal system based on 100s of old scripts and
> the RAID section only allows us to select  drives (not raids), it
> does however use LVM so it would be easier to change the script which
> created the VG as opposed to change the many scripts related to RAID
> creation...

I guess that is your answer then - use LVM over MD.

>
> If there is a real disadvantage in using LVM, then we can manually
> create the RAIDs I guess....

I haven't used LVM in this configuration so I cannot report first
hand experience, but I would be very surprised if LVM did not meet
your needs.

NeilBrown

>
>
>
> I would appreciate any feedback
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