On Sat, 26 Jun 2010 15:12:35 +0200
Piergiorgio Sartor [off-list ref] wrote:

Hi,

quoted

Assuming the code doesn't have any bugs, the reshape will stop, then
immediately restart picking up where it left off.

thanks, that's what I wanted to know.

quoted

You will of course end up with a degraded array

Yes, that was clear.

quoted

It might be nice in these circumstances to abort the reshape and revert back
the the previous number of devices - particularly if it was the new device
that failed.  However that currently isn't supported.

Well, probably as an option, it could be interesting.

Actually, I would be still interested, we already
discussed the topic, on a RAID-5/6 with HDDs of
different size.
This would simplify many things...

quoted

quoted

1)
mdadm --grow ...
mdadm --wait
pvresize

Yes.

quoted

2)
mdadm --grow
pvresize

No.
Until the reshape has completed, the extra space is not available.

There seem to be an issue, here, maybe.

Using the command line:

mdadm --grow /dev/md/vol02 --bitmap=none; mdadm --grow /dev/md/vol02 -n 9 --backup-file=/var/tmp/md125.backup; mdadm --wait /dev/md/vol02; mdadm --grow /dev/md/vol02 --bitmap=internal --bitmap-chunk=128

Note that /dev/md/vol02 is the usual link to /dev/md125,
which should be the same for this scope, I guess.

I got (in two independent tests):

mdadm: Need to backup 2688K of critical section..
mdadm: failed to set internal bitmap.

Re-issuing:

mdadm --wait /dev/md/vol02; mdadm --grow /dev/md/vol02 --bitmap=internal --bitmap-chunk=128

Does wait.

Could it be the devices (being USB) are so slow
that some race condition is uncovered and the
immediate "--wait" after the "--grow" does not work?

Yes, there  is a race here.  The reshape doesn't quite start instantly and so
--wait doesn't notice.
I've added a note to my todo-list to look into this.
For now, a 'sleep 1' between the --grow and the --wait should be enough.

Thanks,
NeilBrown