Assume-clean for md grow

[Chris Webb <chris@arachsys.com>]

I use md arrays made up of slots synthesized through device mapper from
physical storage distributed across a cluster of machines. When these slots
are created, they are guaranteed to be zero-initialised, so I can safely do
mdadm --create --assume-clean to avoid an initial resync.

When I grow the backing slots, the new storage space is also
zero-initialised, so I'd like to be able to do the equivalent of
mdadm --grow --size=max --assume-clean. However, --assume-clean isn't
supported for grow operations.

Is there some way I can tell the kernel driver (perhaps through sysfs) that
the resync is clean/already complete to avoid unnecessary heavy IO on every
grow? I tried things like

  echo idle >/sys/block/mdX/md/sync_action

without success.

Cheers,

Chris.