Code review comment for lp:~spiv/bzr/fetch-spec-everything-not-in-other

In the case of lp:emacs there's no significant performance difference. The HPSS call count, memory consumption and wall clock time are unchanged (aside from noise in measurement). The HPSS request for the initial 'bzr branch' is larger, as it requests every rev named in the tags, not just tip, but that's a drop in the bucket vs. fetching the 239M of lp:emacs.

Also, lp:emacs has no tags pointing to missing revs (or even non-ancestry revs), so "bzr pull" works out identically with the new code vs. bzr.dev: all the revisions named in the source's tags are already present, so nothing changes. The wall clock time for the new code is again the same (within the noise of my measurements).

The main case I'd be concerned about is when the source branch has tags that refer to missing revisions. The target will copy those tags, and then on every update/pull re-request those tags from the source repository. Because those lookups in the revisions index will be misses, the server will end up opening every *.rix file. In the HPSS case that will happen server side, so it probably isn't a big concern there, but it might be an issue if using a dumb remote transport. The solution for affected branches is to delete the broken tags, or fill in the missing revisions. That's probably worth the benefit this gives, which is making tags Just Work in more cases. Branches and tags made with this new bzr version will of course fetch all tags by default, so that scenario won't be a concern for them.

In short: I'm confident. If it copes with a branch with as much history and tags as lp:emacs, it probably copes with everything else.