Sometimes we can see synchronization issue between disks in software RAID. We can verify the consistency of Linux software RAID-disks with reading operations via comparing the corresponding blocks of each disk in the array. If an error occurs, the counter is increased in the /sys/block/mdX/md/mismatch_cnt file.
To see count of mismatch between disks we can use mismatch_cnt:
A manual check or repair can be triggered via sysfs:
echo check >/sys/block/mdX/md/sync_action
echo repair >/sys/block/mdX/md/sync_action
Also we can use watch utility to monitor the progress:
watch cat /proc/mdstat