Linux distributions like Debian or Ubuntu with software RAID (mdadm) run a check once a month (as defined in /etc/cron.d/mdadm).
To check if a test is running, do:
As you can see, it is running, with 90K/sec speed (very overloaded array).
To cancel a running test, use:
Confirm with "cat /proc/mdstat" after that if the check didn't start on a different array.
To start a check manually, do:
Some older kernels may have "sync_action" located elsewhere, i.e. /sys/block/md0/md/sync_action.
Below ones will find the files where you can start/stop the array check: