% systemctl --system cat btrfs-scrub@.service

# /usr/lib/systemd/system/btrfs-scrub@.service
[Unit]
Description=Btrfs scrub on %f
ConditionPathIsMountPoint=%f
RequiresMountsFor=%f

[Service]
Nice=19
IOSchedulingClass=idle
KillSignal=SIGINT
ExecStart=/usr/bin/btrfs scrub start -B %f