Setting up a new linux software raid tonight I noticed some new behavior from
mdadm that I thought I might share quickly. It looks like, as of around version 2.6.4,
mdadm has started creating new raid devices in
auto-read-only mode. In this mode, the array flips to read/write after the first write attempt.
Johnny said that's a good idea, because a lot of people reboot right after creating the array (during an install), so this behavior could prevent aborted syncs. Unfortunately, the array must be writable for recovery to occur, and building up the parity blocks in RAID-5 is considered recovery, so to actually get your raid active and in a non-degraded state, you'll want to either:
mdadm --readwrite /dev/mdX
Or you can go ahead and start creating the filesystem.