Re: expand raid10

[NeilBrown <neilb@suse.de>]

On Wed, 13 Apr 2011 14:34:15 +0200 David Brown <david@westcontrol.com> wrote:

> 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.
> 

There is an interesting idea in here....

Currently if the devices in an md/raid array with redundancy (1,4,5,6,10) are
of difference sizes, they are all treated as being the size of the smallest
device.
However this doesn't really make sense for RAID10-far.

For RAID10-far, it would make the offset where the second slab of data
appeared not be 50% of the smallest device (in the far-2 case), but 50% of
the current device.

Then replacing all the devices in a RAID10-far with larger devices would mean
that the size of the array could then be increased with no further data
rearrangement.

A lot of care would be needed to implement this as the assumption that all
drives are only as big as the smallest is pretty deep.  But it could be done
and would be sensible.

That would make point 2 of http://neil.brown.name/blog/20110216044002#11 a
lot simpler.

NeilBrown
--
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html