I also have one other scenario in mind, though - which is one I've actually come across a number of times: What if a certain DLE due for backup is estimated to be slightly smaller than <runtapes>*<tape size>, and thus dumped to holding disk, but then turns out to be slightly larger?

Wouldn't it be more accurate to say the scenario you ran into previously
was DLE larger than <tape size> because the tape spanning feature was
not available at that time.

Yes. But the key issue remains unchanged.

With the current setup, amanda will obviously run out of tape-space during the original dump and also if you try amflush. And if auto-flush is enabled, the next dump will hit end-of-tape before any of the new dumps have been written, and the next one after that, and so on; this holding disk image will effectively block the tape operation of all the following backups, and eventually, the holding disk will be full, too, so amdump won't be able to do anything at all.

What is different with the tape spanning feature is that you could get the
large DLE to tape by simply increasing runtapes, even if only temporarily.
Thus, no system lockup.

- Toralf