Re: [PATCH 00/18] Assorted md patches headed for 2.6.30

["Keld Jørn Simonsen" <keld@dkuug.dk>]

On Thu, Feb 12, 2009 at 07:28:59AM -0800, Wil Reichert wrote:
> On Thu, Feb 12, 2009 at 2:45 AM, NeilBrown <neilb@suse.de> wrote:
> > On Thu, February 12, 2009 8:53 pm, Keld Jørn Simonsen wrote:
> >> On Thu, Feb 12, 2009 at 08:21:12PM +1100, NeilBrown wrote:
> >>> On Thu, February 12, 2009 7:11 pm, Keld Jørn Simonsen wrote:
> >>> > On Thu, Feb 12, 2009 at 02:10:10PM +1100, NeilBrown wrote:
> >>> >> Comments and testing very welcome.
> >>> >
> >>> > I would rather have functionality to convert raid10 to raid5.
> >>> > raid1 should be depreciated, as raid10,n2 for all purposes is the same
> >>> > but better implementation and performance, and raid10,f2 and raid10,o2
> >>> > are even better.  Nobody should use raid1 anymore.
> >>>
> >>> That is a fairly simplistic view.
> >>
> >> It was also formulated to provoke some thoughts.
> >>
> >>> raid1 supports --write-mostly and --write-behind which raid10 is
> >>> unlikely
> >>> ever to support.
> >>
> >> why?
> >>
> >> Anyway would it not be possible that this functionality be implemented
> >> for raid10,n2?
> >
> > It would be possible, but it might not be sensible.
> >
> > write-mostly and write-behind only really make sense when you have the
> > clear distinction between drives that raid1 gives you.
> > These options don't make sense for raid10 in general.  Only in very specific
> > layouts.
> > If you like, raid1 is an implementation of a specific raid10 layout,
> > where it makes sense to add some extra functionality.
> >
> >>
> >> Some code to grow raid10 would also be desirable. Maybe it is some of
> >> the same operations that need to be applied: getting the old data in,
> >> have it restructured for the new format, in a safe way, and possibly
> >> with the help of an extra disk, or possibly not. It sounds non-trivial
> >> to me too.
> >
> > What particular growth scenarios are you interested it?
> > Just adding a drive and restriping onto that?  i.e keep that
> > same nominal layout but increase 'raid-disks'?
> >
> > That would be quite similar to the raid5 grow operation so it shouldn't
> > be too hard to achieve.
> > A 'grow' which changed the layout (e.g. near to far) would be a lot
> > harder.
> 
> I'd previously seen the wikipedia article regarding Linux RAID10 and
> its mention of the 3 disk case.  Out of academic curiousity, how does
> the 2 disk RAID10 work?  Is it just a matter of have 2 identical
> volumes and reading subsequent stripes from the alteranate drives?  Or
> is the algorithm more complicated?

There are 3 different layouts for raid10: near, far and offset.
Basically raid10 with 2 disks works as RAID 1 - and "near" and linux
raid1 is almost equivalent. far and offset has somewhat different
placements of blocks on the disks, which should lead to faster operation
for some common tasks.

best regards
keld
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