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FAQ: sharpen the restic-interop caveat — restic prune vs any rustic write #144

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FAQ: sharpen the restic-interop caveat — restic prune vs any rustic write #144

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## Can I use rustic with my existing restic repositories?

Yes, you can. rustic uses the same repository format as restic, so you can use
rustic and restic on the same repository. The only thing you have to take care
of is that you don't run prune with restic and rustic at the same time.
rustic and restic on the same repository.

There is one caveat worth stating carefully, because rustic and restic reach
safe concurrent access by *different* mechanisms and the two do not compose:

- **rustic is lock-free.** `prune` marks packs and deletes them only after
`--keep-delete` (default 23h), so a parallel rustic `backup` has that long to
finish. See [Are all operations lock free?](#are-all-operations-lock-free).
- **restic instead takes an exclusive lock** inside the repository, which is
what lets its `prune` delete packs immediately.

**rustic neither takes nor honours restic's lock.** So a `restic prune` running
while *any* rustic command is writing can delete packs that the rustic run has
just written and not yet referenced, leaving the repository failing
`restic check`.

Note this is not limited to prune-versus-prune:

| combination | safe | why |
| ------------------------------------ | ------ | ------------------------------------------------------------ |
| `rustic prune` + rustic `backup` | yes | the `--keep-delete` grace period |
| `restic prune` + restic `backup` | yes | restic's repository lock |
| **`restic prune` + rustic `backup`** | **no** | restic deletes at once, relying on a lock rustic never takes |
| `rustic prune` + restic `backup` | yes | the grace period still applies |

In short: **never run `restic prune` against a repository a rustic command may
be writing to.** If every client is rustic, none of this applies.

## What are the differences between rustic and restic?

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