I attempted to grow an existing RAID6 MD device this week, and ran into the following error when I performed the grow operation:
$ mdadm --grow --raid-devices=5 --backup-file=/tmp/mdadmgrow.tmp
/dev/md0
mdadm: Need to backup 384K of critical section..
mdadm: Cannot set device size/shape for /dev/md0: Invalid argument
It appears the ability to grow a RAID6 device was added in the Linux 2.6.21 kernel, and this feature has yet to be backported to RHEL5 (the mdadm manual page implies that this should work, so I reckon there is a documentation mismatch). If you are enountering this error, you will need to switch to a newer kernel in order to be able to grow RAID6 devices on RHEL5 systems. 2.6.33 worked like a champ, and I hope this issue is addressed when RHEL6 ships.