Re: expand raid10

[David Brown <david@westcontrol.com>]

On 13/04/2011 13:17, NeilBrown wrote:
> On Wed, 13 Apr 2011 13:10:16 +0200 Keld Jørn Simonsen<keld@keldix.com>  wrote:
>
>> On Wed, Apr 13, 2011 at 07:47:26AM -0300, Roberto Spadim wrote:
>>> raid10 with other layout i could expand?
>>
>> My understanding is that you currently cannot expand raid10.
>> but there are things in the works. Expansion of raid10,far
>> was not on the list from neil, raid10,near was. But it should be fairly
>> easy to expand raid10,far. You can just treat one of the copies as your
>> refence data, and copy that data to the other raid0-like parts of the
>> array.  I wonder if Neil thinks he could leave that as an exersize for
>> me to implement... I would like  to be able to combine it with a
>> reformat to a more robust layout of raid10,far that in some cases can survive more
>> than one disk failure.
>>
>
> I'm very happy for anyone to offer to implement anything.
>
> I will of course require the code to be of reasonable quality before I accept
> it, but I'm also happy to give helpful review comments and guidance.
>
> So don't wait for permission, if you want to try implementing something, just
> do it.
>
> Equally if there is something that I particularly want done I won't wait for
> ever for someone else who says they are working on it.  But RAID10 reshape is
> a long way from the top of my list.
>

I know you have other exciting things on your to-do list - there was 
lots in your roadmap thread a while back.

But I'd like to put in a word for raid10,far - it is an excellent choice 
of layout for small or medium systems with a combination of redundancy 
and near-raid0 speed.  It is especially ideal for 2 or 3 disk systems. 
The only disadvantage is that it can't be resized or re-shaped.  The 
algorithm suggested by Keld sounds simple to implement, but it would 
leave the disks in a non-redundant state during the resize/reshape. 
That would be good enough for some uses (and better than nothing), but 
not good enough for all uses.  It may also be scalable to include both 
resizing (replacing each disk with a bigger one) and adding another disk 
to the array.

Currently, it /is/ possible to get an approximate raid10,far layout that 
is resizeable and reshapeable.  You can divide the member disks into two 
partitions and pair them off appropriately in mirrors.  Then use these 
mirrors to form a degraded raid5 with "parity-last" layout and a missing 
last disk - this is, as far as I can see, equivalent to a raid0 layout 
but can be re-shaped to more disks and resized to use bigger disks.




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