Neil,

The write behind value does not get converted to/from little endian 
which causes write behind not to work on big endian machines (RUN ARRAY 
fails with invalid argument error):

# mdadm -B /dev/md0 -l1 -n2 --write-behind=256 /dev/sdb10 --write-mostly 
/dev/nbd0 --bitmap=/bitmaps/test5
mdadm: RUN_ARRAY failed: Invalid argument

# tail /var/log/messages
Oct  6 13:14:48 caspian kernel:   daemon sleep: 5s
Oct  6 13:14:48 caspian kernel:      sync size: 1001456 KB
Oct  6 13:14:48 caspian kernel: max write behind: 65536
Oct  6 13:14:48 caspian kernel: md0: failed to create bitmap (-22)


Attached patch for mdadm fixes this.

Thanks,
Paul