From c99fb589097d96b4e10cd8b137f72881cdb93118 Mon Sep 17 00:00:00 2001 From: Joey Hess Date: Sun, 6 Nov 2011 15:18:45 -0400 Subject: merge: Use fast-forward merges when possible. Thanks Valentin Haenel for a test case showing how non-fast-forward merges could result in an ongoing pull/merge/push cycle. While the git-annex branch is fast-forwarded, git-annex's index file is still updated using the union merge strategy as before. There's no other way to update the index that would be any faster. It is possible that a union merge and a fast-forward result in different file contents: Files should have the same lines, but a union merge may change their order. If this happens, the next commit made to the git-annex branch will have some unnecessary changes to line orders, but the consistency of data should be preserved. Note that when the journal contains changes, a fast-forward is never attempted, which is fine, because committing those changes would be vanishingly unlikely to leave the git-annex branch at a commit that already exists in one of the remotes. The real difficulty is handling the case where multiple remotes have all changed. git-annex does find the best (ie, newest) one and fast forwards to it. If the remotes are diverged, no fast-forward is done at all. It would be possible to pick one, fast forward to it, and make a merge commit to the rest, I see no benefit to adding that complexity. Determining the best of N changed remotes requires N*2+1 calls to git-log, but these are fast git-log calls, and N is typically small. Also, typically some or all of the remote refs will be the same, and git-log is not called to compare those. In the real world I expect this will almost always add only 1 git-log call to the merge process. (Which already makes N anyway.) --- doc/bugs/making_annex-merge_try_a_fast-forward.mdwn | 6 ++++++ 1 file changed, 6 insertions(+) (limited to 'doc') diff --git a/doc/bugs/making_annex-merge_try_a_fast-forward.mdwn b/doc/bugs/making_annex-merge_try_a_fast-forward.mdwn index a2bd8c747..41a5a2a58 100644 --- a/doc/bugs/making_annex-merge_try_a_fast-forward.mdwn +++ b/doc/bugs/making_annex-merge_try_a_fast-forward.mdwn @@ -27,3 +27,9 @@ But as sometimes annex-merge takes time, it would probably be worth it > > Although, perhaps fast-forward merge would use slightly > less space. --[[Joey]] + +>> To avoid the ladder-merge between two repositories described at +>> , seems a fast-forward should be detected and +>> written to git, even if the index is still updated the current way. +>> [[done]] +>> --[[Joey]] -- cgit v1.2.3