[ralph]

I often think it would be useful for a filesystem to provide a digest of a file's content. It's the FS that knows when the file's content changes and the digest is out of date, and it also only needs to recalculate if anything asks. It wouldn't necessarily have to read all the bytes of the file to re-calculate; it may be a hierarchical digest where much of the existing, stored, labour can be re-used.

[wezfurlong]

What would be a good start would be portable and reliable snapshots with differencing managed by the kernel. Some filesystems offer this capability but it is not available to us in this particular circumstance.