[reycharles]

I helped build a tar-backed filesystem for MirageOS[0]. It is definitely easiest to make it append-only. I did add in-place renaming, updates (of file content), pre-allocating a file and removing the last file.

We also scan the filesystem on boot and keep a mapping in-memory of file names to file metadata and block offsets, and update this memory representation when modifying the on-disk tar filesystem. If you have a lot of small files this is maybe not a great idea as you will be storing most in memory, though.

For the purpose we had in mind it worked fine: A content addressable mirror of package archives. The archives exist somewhere on the web and the package file has a link to the archive and a checksum. We can then download the package and store it by the checksum. This gives integrity that the tar filesystem does not offer. Removing the last file works great if you only have one download job. Otherwise if a download fails and it is not the latest file you can rename it to `failed/<hash>-<random>` and do garbage collection in the filesystem at a later point.

[0]: https://github.com/mirage/ocaml-tar/blob/main/mirage/tar_mir...

Update: interestingly, this was motivated after trying to use a (mostly) LittleFS-compatible filesystem which unfortunately didn't work very well for us at the time (bugs, poor performance).