I’ve been thinking through how I’d write this. With so many files it’s probably worth using sqlite, and then I can match them up by joining on the hash. Deletions and new files can be found with different join conditions. I found a tool called ‘hashdeep’ that can checksum everything, though for incremental runs I’ll probably skip hashing if the size, times, and filename haven’t changed. I’m thinking nushell for the plumbing? It runs everywhere, though they have breaking changes frequently. Maybe rust?

ZFS checksums are done at the block level, and after compression and encryption. I don’t think they’re meant for this purpose.