Re: Swapping a disk without degrading an array

[Goswin von Brederlow <goswin-v-b@web.de>]

Gabor Gombas <gombasg@sztaki.hu> writes:

> On Sun, Jan 31, 2010 at 04:34:03PM +0100, Asdo wrote:
>
>> 1/ The live-spare gets filled of data without recording anything on
>> any superblocks. If there is a power failure and reboot, the new MD
>> will know nothing about this. The process has to be restarted.
>
> IMHO MD must know about the copy and it must know not to use the new
> device before the copying is completed. Otherwise after a reboot mdadm
> may either import the new half-written spare instead of the real one if
> the superblock is already copied, or other tools like LVM may start
> using the new half-written spare instead of the RAID if the MD
> superblock is still missing.
>
> Gabor

No that is exactly what he means to avoid.

His suggestion is that at the start the metadata area of the life-spare
is kept as is, being a simple unused spare. Only the in-memory data
records that it actualy is a live-spare and only the data part of the
device is mirrored.

Then at the end you remove the old disk, add the live-spare and record
the change in the metadata of all drives in an semi atomic way. If
anything interrupts the operation before this the live-spare will still
be recorgnised as normal spare when the raid is reassembled.

MfG
        Goswin