On Monday May 21, brugolsky@xxxxxxxxxxxxxxxxxxxxxxxxx wrote:
> Neil,
>
> What seems desirable to me is a way to take a new (larger) spare drive and
> add it to a RAID1 for a particular RAID 4/5/6 component, and then when
> it's sync'd, replace the now redundant small drive with another larger
> drive. Wash, rinse, repeat. This way the array is never degraded.
> Though I imagine that this particular arrangement doesn't have the
> benefit of the stripe rewrite when encountering a latent error on the
> drive that is being migrated. [Presumably the failing addresses could
> be cycled through the check from userland though, by doing a read above
> the stacked RAID.]
>
> One could start a RAID 4/5/6 array over a degraded RAID1 for each
> component, (i.e., a degraded RAID1).
>
> I haven't been following the metadata changes closely. Is it possible
> to do this with external MD metadata? It can also be done with
> device-mapper, but dm-mirror is very immature compared to MD RAID1.
>
> Comments?
This doesn't really have anything to do with the metadata used - it is
primarily an implementation issue (though you would need to be careful
picking up the pieces after a crash).
If we could freeze an array (so that all writes block), then we could
do much of what you suggest:
- freeze the array
- remove the target device
- create a raid1 of the target and the new
- re-add the raid1
- unfreeze the array.
The issue of dealing with read errors on the target device is much
more awkward to deal with. The approach that seems right to me is:
- create a raid1 variant which does a passive resync: When the
next-needed block is read or written, write it to the second
device and advance the "next-needed" pointer.
- Get this raid1 to simply return read errors (which might be OK
already) so that a read-error won't be fatal. But a read request
that be behind the "next-needed" pointer gets served from the
second device if the first does fail.
- Implement a 'check-one-disk' operation on raid5 (and others) so
that instead of reading all devices, it just reads all through
one. If this one is really a raid1-variant, doing that read will
effect a resync on the raid1, and any read error will be handled
correctly.
So it is all quite possible, and I agree that it could be valuable.
It just needs someone to do it, and work out all the fine details.
Anyone want to try some coding ????
NeilBrown
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