An Amanda gotcha with dumps to disk
One of the configurations that Amanda supports these days is dumping to disk instead of to tape. The leading way to do this is to use your disk space as virtual tapes, using the 'file' output driver (per here).
When you do this, it is vital to remember that to Amanda, these are tapes. Very fast tapes, but tapes. And they have all of the limitations of tape. Such as that only one thing can be written to them at once.
So if you do not set up a holding disk in addition to your disk space for the 'tapes' (because why would you need a holding disk when you're dumping to disk anyways), Amanda will behave just like it would with a real tape: it will serialize all your backups, because they have to go to directly to tape.
(It is surprisingly hard to notice this, because the daily Amanda reports do not mention that they are dumping directly to tape and does not have the start time of the various dumps, which would have let us see this clearly. However, amstatus will tell you that only one dumper was ever used, which is a dead giveaway, and you can run amstatus even after amdump finishes.)