This is very difficult to test so I'm looking for some advice with this one!
Essentially we do a few "rollovers" a year which backup 1250+ courses and restore them into a new Moodle installation.
This year, with our fancy new beanstalk adhoc task queue thing, we have 32 processes on backup duty and 32 on restore.
What we've noticed is a lot of failures when backing up in parallel like this, because the backup is running as MODE_GENERAL with no users, which sends the file to the user's backup folder (in this case "admin") and does so with an ID of 0. We get this:
Task failed: Can not create file "5/user/backup/0/backup.mbz"
I've "fixed" this by changing "itemid" to the context's ID, so each filename will be unique and therefore not conflict / can run in parallel.
As for core Moodle, I'm not sure where this might be a problem but I don't see why itemid should be 0 so I'm hoping the fix makes sense anyway...