I also have a scenario where this issue bites me. A rather large backup set (1.6 TB) on a node with an 8 GB root partition and a 4 TB /home partition. The /root/.cache/duplicity folder fills up the root disk with the sigtar/difftar files. After changing /root/.cache to a symlink to /home/tmp/.cache and the TMPDIR to /home/tmp the backup now runs without issues.
I also have a scenario where this issue bites me. A rather large backup set (1.6 TB) on a node with an 8 GB root partition and a 4 TB /home partition. The /root/. cache/duplicity folder fills up the root disk with the sigtar/difftar files. After changing /root/.cache to a symlink to /home/tmp/.cache and the TMPDIR to /home/tmp the backup now runs without issues.