Philip and I had an interesting conversation with some users this evening, and I'm just archiving my brain dump here.
These users have a large repository with a large number of branches in the /branches directory (~35k). We described the well-known phenomenon in which directories aren't deltified on commit, and thus cause the repository to have very large revisions, even when the actual content changes are fairly small. This is due to bubble up and having to re-write the entire directory list of the /branches directory. Philip recalled a time several years ago when he enabled directory deltification, but the performance was awful, and we've never released it. In our discussion, we mentioned that directory deltification may be better performing now, especially in light of the imminent merge of the diff-bytes-optimizations branch. In the case of a bubble-up directory modification, the prefix and suffix matching would simplify the problem space, leaving a very small diff. The only trouble with the above theory is if directory entry lists are stored in a hash, and are serialized in an unordered manner, thus negating any benefits prefix-scanning would provide (and potentially causing the horrific delta performance in the first place). Anyway, that was the kernel of our discussion. I haven't dug around in the code to determine how much of it is true or not, but if anybody wants something to do, this might be interesting. -Hyrum
