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When pruning and repacking a repository that has an alternate object store configured, we may traverse a large number of objects in the alternate. This serves no purpose, and may be expensive to do. A longer explanation is below. Commits d3038d2 and abcb865 taught prune and pack-objects (respectively) to treat "recent" objects as tips for reachability, so that we keep whole chunks of history. They built on the object traversal in 660c889 (sha1_file: add for_each iterators for loose and packed objects, 2014-10-15), which covers both local and alternate objects. In both cases, covering alternate objects is unnecessary, as both commands can only drop objects from the local repository. In the case of prune, we traverse only the local object directory. And in the case of repacking, while we may or may not include local objects in our pack, we will never reach into the alternate with "repack -d". The "-l" option is only a question of whether we are migrating objects from the alternate into our repository, or leaving them untouched. It is possible that we may drop an object that is depended upon by another object in the alternate. For example, imagine two repositories, A and B, with A pointing to B as an alternate. Now imagine a commit that is in B which references a tree that is only in A. Traversing from recent objects in B might prevent A from dropping that tree. But this case isn't worth covering. Repo B should take responsibility for its own objects. It would never have had the commit in the first place if it did not also have the tree, and assuming it is using the same "keep recent chunks of history" scheme, then it would itself keep the tree, as well. So checking the alternate objects is not worth doing, and come with a significant performance impact. In both cases, we skip any recent objects that have already been marked SEEN (i.e., that we know are already reachable for prune, or included in the pack for a repack). So there is a slight waste of time in opening the alternate packs at all, only to notice that we have already considered each object. But much worse, the alternate repository may have a large number of objects that are not reachable from the local repository at all, and we end up adding them to the traversal. We can fix this by considering only local unseen objects. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
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Git - the stupid content tracker
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