vegetable26

edit: author here

Yah that's a good point. We use a similar copy-on-write implementation as many other storage systems (such as Iceberg) that offer time-travel, branching, etc... where you have snapshots represented by pointers to immutable snapshot-layer or delta files (or however else you want to call them).

In our case, we wanted to provide customers with a very similar/seamless experience of using DuckDB on MotherDuck as they would with local DuckDB. To provide this parity we needed the ability to extend DuckDB's native storage to support these capabilities (time-travel, branching, etc...). This led us to implementing Differential Storage.

edit: author here

Our current FUSE implementation is specific to DuckDB and does make (as well as validate) some assumptions about DuckDB's access patterns to the underlying files. In this case - we know that currently DuckDB always truncate the WAL on successful checkpoint - which triggers a Differential Storage snapshot under the hood.

We are working on future-proofing this setup by removing our reliance on some of these assumptions and having our FUSE implementation act much more like a generic FS moving forward.

Though in general the truncation of the WAL is still usually a good time to snapshot the database, as the truncation means that the only "state" needed to reconstruct the current database is completely captured in the database file.