> There's no extra storage because it's already storing checksums.

At the database level (i.e. not just the WAL)? Are you sure?

> What I'm saying is that a fletcher is strictly worse than a CRC here.

I can't speak to the performance differences, but the only thing SQLite really needs the checksum to do is to expose partial writes, both due to reordered sector writes and partial intra-sector writes. (The former could also be solved by just using an epoch counter, but the latter would require some tricky write formats, and a checksum nicely addresses both).

In both cases, there's really nothing to recover: CRC won't catch an entire missing sector, and almost no partially written sectors (i.e. unless the failure somehow happens in the very last bytes of it, so that the ratio of "flipped" bits is low enough).

Just talking about the WAL. The database does have a page level checksum with the VFS extension, but it's optional.