After I upgraded a bunch of servers, one of them started sending me warnings like this once per week, when cron.weekly ran.
/etc/cron.weekly/99-raid-check: WARNING: mismatch_cnt is not 0 on /dev/md4
Steve Tendall posted about this on the CentOS list stating this warning is telling us that there are unsynchronized blocks in the array. This doesn’t sound good to me. So I followed his advice to repair the array, which worked for me too. This is the relevant quote from his post:
The repair is to rebuild the raid:# echo repair >/sys/block/md<#>/md/sync_action
…which does not reset the count, but if you force a check after the rebuild is complete:# echo check >/sys/block/md<#>/md/sync_action
…then the count should return to zero.