There was a kernel bug that got this wrong, so better check for it.
Signed-off-by: NeilBrown <neilb@suse.de>
mdadm -CR $md0 -l6 -n5 $dev0 $dev1 $dev2 $dev3 $dev4
check wait
check raid6
+testdev $md0 3 19456 512
mdadm -G $md0 -l5
check wait
check raid5
+testdev $md0 3 19456 512
mdadm -G $md0 -l0
check wait
check raid0
+testdev $md0 3 19456 512
mdadm -G $md0 -l5 --add $dev3 $dev4
check wait
check raid5
check algorithm 2
+testdev $md0 3 19456 512
mdadm -G $md0 -l 6
check wait
check raid6
check algorithm 2
+testdev $md0 3 19456 512