Too big space usage by archive dir

You can remove the entire archive dir after the backup if you wish. It will be downloaded the next time you use duplicity, so no harm done. The archive dir (cache dir) is merely keeping local the things that it would normally have to download in the normal process of doing incremental backups. You probably need to specify an archive dir in a different directory, like this: "duplicity --archive-dir=/tmp". That way, the space goes away on the next boot, but better yet, its available if you need to do another backup, or if you need to restart an interrupted backup.

Checkpoint/restart (in 0.6 and up) uses the data in the archive dir to enable restart if duplicity is interrupted in the middle of a run. Since the data needs to be in permanent visible files for that to occur, I just left the files in a more formal location than some random temp directory.

Bottom line is that duplicity is not using any more room during the run than it had previously, its just leaving more behind on the source system, and maybe putting it in a different place, which makes it more visible. The tradeoff is going to be worth it in terms of restart capabilities and less network overhead.