Comment by Intralexical

> As I'm sure the author is aware, Restic will do hash-based chunking so that similar files can be efficiently be backed up.

> Even if the order of the mbox's messages are ~random, Restic's delta updates will forego large attachments.

I forget the exact number, but the rolling hashes for Restic and Borg are tuned to produce chunks sizes on the order of an entire megabyte.

Which means attachment file sizes need to be many megabytes in order for Restic to be much use, since the full chunk has to fall within the attachment. — You'd lose 0.5MB at both ends of each attachment on average, so a 5MB file would only be 80% deduped.

Nothing against Restic, but it's tuned for file-level backup, and I'm sure it wouldn't be as performant if it used chunks that were small enough to pick apart individual e-mails.

I suggested the author check out ZPAQ, which has a user-tunable average fragment size, and is arguably even simpler than Restic.

The ZPAQ file can then itself be efficiently backed up by Restic.