quoted

Another thing is waiting during reshape for metadata update on

MD_CHANGE_DEVS flag.

quoted

To roll reshape I've added the following code (instead calling

md_ubdate_sb()):

Yes, there is a real issue there...

I don't think we ever need the kernel to wait for an external metadata
handler
to respond to device changes (apart from failure which is handled
separately).
So maybe the best thing is to guard all settings of MD_CHANGE_DEVS with
if (mddev->persistent)

I think that would be best, but I've make a note to review that later.

Neil,
from what I see in the raid5.c/md.c "native" code uses MD_CHANGE_DEVS
during the reshape if it reaches special points when metadata
write is really needed to update the reshape checkpoint.
In reshape_request():
	/* Cannot proceed until we've updated the superblock */
	..
	set_bit(MD_CHANGE_DEVS, mddev->flags)

In md_check_recovery() we have:
	if (mddev->flags) 
		md_update_sb()

Couldn't we follow this logic with MD_CHANGE_DEVS for external metadata?
If not, how to detect the need for migration checkpoint update?

Thanks,
Maciek