The compressed zip defeated restic's content-defined chunking: each
day's dump looked completely different to restic even when the repo
content barely changed, causing repo growth of ~full dump size (821 MB
at last measurement) every day until forget/prune aged snapshots out.
Uncompressed tar is dedup-friendly. After the first snapshot, daily
incrementals cost only the actual new-data delta — typically a few
MB for an active repo.
Tradeoff: stage file on the client host is ~2-3x the zip size while
the dump is in flight, but that's transient (purged at the start of
each run). Repo-side storage is much smaller over time.
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