more scheduler docs

howto/backup.md

@@ -1242,7 +1242,15 @@ also from `puppet-crontab` which will:
    (`recycle=1`) and older than 4 months
 
 It doesn't actually seem to purge old volumes per say: something else
-seems to be responsible from marking them as `Purged`.
+seems to be responsible from marking them as `Purged`. This is
+(possibly?) accomplished by the Director, thanks to the `Volume
+Retention` settings in the storage jobs configurations.
+
+All the above run on the Director. There's also a cron job
+`bacula-unlink-removed-volumes` which runs daily on the storage server
+(currently `bungei`) and will garbage-collect volumes that are not
+referenced in the database. Volumes are removed from the storage
+servers 60 days after they are removed from the director.
 
 ## Issues