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I use Carbon Copy Cloner for my primary backup to an external SSD, and I am very happy with that. A particularly interesting feature is the use of APFS snapshots on the target hard drive for maintaining history.

However, following the 3-2-1 principle, I also use Arq 7 to backup to a cloud service (as CCC does not support cloud backends), and I want to backup to a makeshift NAS using a Raspberry Pi 4. However, backing up to an SMB share is unacceptably slow, and there is very limited support for keeping history through something they call CCC SafetyNet.

Yesterday I spent all day trying to find the perfect rsync incantation for this, and while performance improves considerably compared to SMB, I can't manage to complete a backup without freezing rsync and having to reboot the Raspberry Pi 4. Thus I have given up on rsync, as a backup tool is something that you need to unconditionally trust, and I can't trust it anymore. Plus this option does not allow to keep history at all.

Researching alternatives, I have come across many options such as Borg, Restic, Duplicacy and Arq itself. While these options seem interesting, the main drawback for me is that they appear to implement a filesystem on top of the filesystem itself, so as to implement their desirable features such as compression, integrity checking, encryption, deduplication, keeping history, etc. As such, your files are not directly accessible in the target hard drive, but rather you have to go through these programs to restore your data. This leaves me a little uneasy, as it adds an extra layer that may fail when you need to restore the backup.

In my makeshift NAS, I have formatted my target hard drive as ZFS, which already implements many of the desirable features of these apps, such as compression, encryption, integrity checking, deduplication and snapshots (for keeping history). It seems to me the backup problem could be easily solvable by an app that works somewhat like rsync to efficiently transfer the backup over the network to the target drive, and then creates a ZFS snapshot to keep history, while automatically achieving the other features through their ZFS native counterparts. As a bonus, files would be directly and transparently accessible via e.g. an SMB share if needed, without post-processing to extract them from the opaque data structures that Borg/Restic/Duplicity use.

Is there a backup app that takes this simpler route of offloading these features to the filesystem as I described? Free is ideal, and I don't mind command-line-only apps, but I will entertain reasonably priced options (up to say ~$50), especially if they include a GUI.

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zfsbackup appears to be a set of scripts that does exactly what I want. I will investigate it, particularly whether it is more resilient to rsync errors.

EDIT: unfortunately, after a few hours of trying to make it work, it appears too many changes would be required for it to work with a macOS client, and so I've given up. I'd encourage anyone sufficiently interested in this idea, and with free time to fix its issues, to investigate it (and ideally send pull requests to the author, or create a fork).

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