API for checkpoint / snapshot / backup system

[thedodd]

Use Case:

Rocks equivalent:

• https://github.com/facebook/rocksdb/wiki/Checkpoints
• https://docs.rs/rocksdb/0.15.0/rocksdb/checkpoint/struct.Checkpoint.html

The primary use case here is exactly as described in the Rocks use case. Full & incremental backups. In my specific use case, I am building a distributed data storage system which uses async-raft for consensus, which includes a protocol where snapshots are sent to new nodes to bring them up-to-speed. Good stuff.

Proposed Change:

The API I am envisioning is as follows.

• sled::DB gets a new method checkpoint(path: impl AsRef<Path>), which does the work of generating a new DB checkpoint which will be written to the given path.
• Given the nature of checkpoints, it seems logical that this method only be exposed on the DB type and not on Trees (maintainers would definitely know better on this one).
• Some discussion is merited around incremental checkpoints / backups and what patterns would be best for this.

Who Benefits From The Change(s)?

Anyone and everyone looking for full / incremental backups, and the various use cases which emerge from that capability.

Alternative Approaches

Instead of calling this new API method checkpoint, we could call it backup.

[ckaran]

@thedodd Take a look at https://github.com/spacejam/sled/issues/587#issuecomment-580276392 as well as the rest of that issue. Can we combine all of the ideas into a single new feature request?