I've been evaluating a ZFS-on-NBD based backup system for several weeks. I've now made a decision if I want to adopt this new setup, or stick with S3QL.
To make it short, I will drop my experiments with ZFS and continue using S3QL.
The main reasons for this are that the advantages of using ZFS are not as big as I had hoped, and that I discovered additional problems with the setup. But let me start with the positive things …