Logging into one of my systems, I saw the following in /proc/mdstat this morning.
Personalities : [raid1]
md0 : active raid1 sdc1 sdb1
976559104 blocks [2/2] [UU]
[=========>...........] resync = 45.7% (446333440/976559104) finish=489.8min speed=18037K/sec
Unaware of a power failure or disk failure (these are pretty new disks) I did some digging. Looking through /var/log/daemon.log, I found these entries:
Mar 1 01:06:02 chewbacca mdadm: RebuildStarted event detected on md device /dev/md0
Mar 1 04:49:03 chewbacca mdadm: Rebuild20 event detected on md device /dev/md0
Mar 1 08:43:03 chewbacca mdadm: Rebuild40 event detected on md device /dev/md0
After some googling around, I found that on the first Sunday of every month at 1:06am, an array check (/usr/share/mdadm/checkarray) is run on Debian and Ubuntu systems. (see /etc/cron.d/mdadm)
For some reason it is listed in /proc/mdstat as a rebuild even though it is really a read-only operation to check the health of the array. Whew! Now I can go get ready for church.