2011-07-07 11:08:35 +00:00
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While merging the git-annex branch, annex-merge does not end up in a fast-forward even when it would be possible.
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But as sometimes annex-merge takes time, it would probably be worth it
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(but maybe I miss something with my workflow...).
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2011-07-10 19:37:29 +00:00
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> I don't think a fast-forward will make things much faster.
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>
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> git-annex needs its index file to be updated to reflect the merge.
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> With the union merge it does now, this can be accomplished by using
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> `git-diff-index` to efficiently get a list of files that have changed,
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> and only merge those changes into the index with `git-update-index`.
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> Then the index gets committed, generating the merge.
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>
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> To fast-forward, it would just reset the git-annex branch to the new
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> head of the remote it's merging to. But then the index needs to be
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> updated to reflect this new head too. To do that needs the same method
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> described above, essentially (with the difference that it can replace
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> files in the index with the version from the git-annex branch, rather
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> than merging in the changes... but only if the index is known to be
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> already committed and have no other changes, which would require both
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> an attempt to commit it first, and
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> locking).
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>
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> So will take basically the same amount of time, except
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> it would not need to commit the index at the end of the merge. The
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> most expensive work is the `git-diff-index` and `git-update-index`,
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> which are not avoided.
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>
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> Although, perhaps fast-forward merge would use slightly
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> less space. --[[Joey]]
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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.)
2011-11-06 19:18:45 +00:00
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>> To avoid the ladder-merge between two repositories described at
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>> <http://sprunge.us/LOMU>, seems a fast-forward should be detected and
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>> written to git, even if the index is still updated the current way.
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>> [[done]]
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>> --[[Joey]]
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