Comment 50 for bug 1852747

Everything work as expected:

# systemctl status mdcheck_start.timer
● mdcheck_start.timer - MD array scrubbing
     Loaded: loaded (/lib/systemd/system/mdcheck_start.timer; enabled; vendor preset: enabled)
     Active: active (running) since Tue 2020-09-29 09:25:28 +07; 5 days ago
    Trigger: n/a
   Triggers: ● mdcheck_start.service

# systemctl status mdcheck_start.service
● mdcheck_start.service - MD array scrubbing
     Loaded: loaded (/lib/systemd/system/mdcheck_start.service; static; vendor preset: enabled)
     Active: activating (start) since Sun 2020-10-04 06:40:59 +07; 5h 17min ago
TriggeredBy: ● mdcheck_start.timer
   Main PID: 3696570 (mdcheck)
      Tasks: 2 (limit: 9481)
     Memory: 1.7M
     CGroup: /system.slice/mdcheck_start.service
             ├─3696570 /bin/bash /usr/share/mdadm/mdcheck --duration 6 hours
             └─3724586 sleep 120

Oct 04 06:40:59 fatman systemd[1]: Starting MD array scrubbing...