/etc/cron.weekly/99-raid-check: WARNING: mismatch_cnt is not 0 on /dev/md0
if uncounted the title's warning in weekly check. We have to do the following to repair and check the array.
echo repair >/sys/block/md0/md/sync_action
echo check >/sys/block/md0/md/sync_action
Ref: http://www.centos.org/modules/newbb/viewtopic.php?topic_id=23164&forum=37 Apparently, a non-zero number is common on RAID-1 and RAID-10 due to various (harmless?) issues like aborted writes in a swap file. So my take on all of that is, if you see it on RAID-5 or RAID-6, you should worry. But if it's on an array with memory mapped files or swap files/partitions that is RAID-1 or RAID-10, it's less of a worry.