ZitatSo background scub is enabled by default? But when initiated manually (or via cronjob) the -B option must be removed?
The command line parameters can be found here:
https://btrfs.wiki.kernel.org/index.php/Manpage/btrfs-scrub
When I run the command manually, which is only after I do something with the btrfs settings, I don't use the B option, and then I use the status command with watch.
In any case, the referenced maintenance scripts insert the scrub into cron.monthly and the heart of it contains:
-Bd keeps the task running in the cron job (matches what you have) and prints out the stats at the end, which is what you want. The $ioprio sets the -c and -n flags based on the configuration settings, and $readonly sets the readonly flag (which I don't see in the manual so have no idea if it does anything other than what its name implies). If you configure for "auto" the the command is run for each btrfs disk found (the $MNT parameter).
The default configuration does a weekly balance and a monthly balance/scrub .
I don't use defrag (and it's not enabled by default) or trim. You definitely don't need snapshotting.