distributed substitutes: file slicing

[Csepp]

I have a question / suggestion about the distributed substitutes
project: would downloads be split into uniformly sized chunks or could
the sizes vary?
Specifically, in an extreme case where an update introduced a single
extra byte at the beginning of a file, would that result in completely
new chunks?

An alternative I've been thinking about is this:
find the store references in a file and split it along these references,
optionally apply further chunking to the non-reference blobs.

It's probably best to do this at the NAR level??

Storing reference offsets is already something that we should be doing to
speed other operations up, so this could tie in nicely with that.