Recently I received (through logwatch) several weekly reports about a mismatch in synchronized block on my md0 (/boot) device :
WARNING: mismatch_cnt is not 0 on /dev/md0
`cat /proc/mdstat` was normal though.
A `echo repair >/sys/block/md0/md/sync_action` followed by a `echo check >/sys/block/md0/md/sync_action` seems to have corrected it. Now `cat /sys/block/md0/md/mismatch_cnt` returns 0 ...