Every 4 weeks the software raid will be checked for its integrity using the mdadm_checkarray function. This is a read only process unless bad data is discovered. If bad data is discovered, the function will try to rewrite the data from other disks in the array.
You will recieve an email alert that this process has started if email alerts are enabled.
For more information, see here http://www.thomas-krenn.com/en/wiki/Mdadm_checkarray