status of raid 4/5 disk reduce

I'm curious as to the status of the ability to reduce the number of disks in a RAID 4/5 array.  I would like the ability to reshape a 4 disk raid4/5 to a 3 disk raid4/5 for flexibility.

here is what I want to do....
$ sudo mdadm /dev/md0 --fail /dev/disk4 --remove /dev/disk4
mdadm: set /dev/disk4 faulty in /dev/md0
mdadm: hot removed /dev/disk4
$ sudo mdadm --grow /dev/md0 -n3
mdadm: /dev/md0: Cannot reduce number of data disks (yet).

I know this capability is missing in the md driver.  What is needed to make it work and is anyone currently working on it?

Regards,

-- 
Mike Brancato, CISSP