Re: RAID60 - MD / LVM

[mark delfman]

Thank you Neil, I appreciate you taking the time to help and advise.

Mark


On Thu, Jul 23, 2009 at 12:06 AM, NeilBrown<neilb@xxxxxxx> wrote:
> 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
>> RAID6’s 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 RAID6’s?
>
> 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 post’s RE: RAID10, then I ‘suspect’ that the
>> MD RAID0 code has some understanding of or tuning for nested RAID’s,
>> 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 100’s 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 RAID’s 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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>
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