<SNIP>

Note that if your md device is not in a read-only mode that
the member states may be changing underneath you as you run
the above command. Therefore, you should either stop the
device then run the commands, or at least have the device in
a read-only mode first.

-BR

I have just tried this - i umounted all LV and then deactivated the VG. I set to read-only but now any attempt to echo check > sync_action results in

'write error: device or resource busy'

any clues/