231 lines
10 KiB
Markdown
231 lines
10 KiB
Markdown
This page's purpose is to collect and explore plans for a future
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annex.version 6.
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There are two major possible changes that could go in v6 or a later
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version that would require a hard migration of git-annex repositories:
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1. Changing .git/annex/objects/ paths, as appear in the git-annex symlinks.
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2. Changing the layout of the git-annex branch in a substantial way.
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## object path changes
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Any change in this area requires the user make changes to their master
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branch, any other active branches. Old un-converted tags and other
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historical trees in git would also be broken. This is a pretty bad user
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experience. (And it bloats history with a commit that rewrites everything
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too.
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For this reason, any changes in this area have been avoided, going all the
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way back to v2 (2011).
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> git-annex had approximately 3 users at the
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> time of that migration, and as one of them, I can say it was a total PITA.
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--[[Joey]]
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So, there would need to be significant payoffs to justify this change.
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Note that changing the hash directories might also change where objects are
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stored in special remotes. Because repos can be offline or expensive to
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migrate (or both -- Glacier!) any such changes need to keep looking in the
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old locations for backwards compatability.
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Possible reasons to make changes:
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* It's annoyingly inconsistent that git-annex uses a different hash
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directory layout for non-bare repository (on a non-crippled filesystem)
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than is used for bare repositories and some special remotes.
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Users occasionally stumble over this difference when messing with
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internals. The code is somewhat complicated by it. In some cases,
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git-annex checks both locations (eg, a bare repo defaults to xxx/yyy
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but really old ones might use xX/yY for some keys).
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The mixed case hash directories have caused trouble on case-insensative
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filesystems, although that has mostly been papered over to avoid
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problems.
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* The hash directories, and also the per-key directories
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can slow down using a repository on a disk (both SSD and spinning).
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<https://github.com/datalad/datalad/issues/32>
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Initial benchmarks suggest that going from xX/yY/KEY/OBJ to xX/yY/OBJ
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directories would improve speed 3x.
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Presumably, removing the yY would also speed it up, unless there are too
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many objects and the filesystem gets slow w/o the hash directories.
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## git-annex branch changes
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This might involve, eg, rethinking the xxx/yyy/ hash directories used
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in the git-annex branch.
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Would this require a hard version transition? It might be possible to avoid
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one, but then git-annex would have to look in both the old and the new
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place. And if a un-transitioned repo was merged into a transitioned one,
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git-annex would have to look in *both* places, and union merge the two sets
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of data on the fly. This doubles the git-cat-file overhead of every
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operation involving the git-annex branch. So a hard transition would
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probably be best.
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Also, note that w/o a hard transition, there's the risk that a old
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git-annex version gets ahold of a git-annex branch created by a new
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git-annex version, and sees only half of the story (the un-transitioned
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files). This could be a very confusing failure mode. It doesn't help that
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the git-annex branch does not currently have any kind of
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version number embedded in it, so the old version of git-annex doesn't even
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have a way to check if it can handle the branch.
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Possible reasons to make changes:
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* There is a discussion of some possible changes to the hash directories here
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<https://github.com/datalad/datalad/issues/17#issuecomment-68558319> with a
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goal of reducing the overhead of the git-annex branch in the overall size
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of the git-annex repository.
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Removing the second-level hash directories might improve performance.
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It doesn't save much space when a repository is having incremental changes
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made to it. However, if millions of annexed objects are being added
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in a single commit, removing the second-level hash directories does save
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space; it halves the number of tree
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objects[1](https://github.com/datalad/datalad/issues/17#issuecomment-68759754).
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Also,
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<https://github.com/datalad/datalad/issues/17#issuecomment-68569727>
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suggests using xxx/yyy.log, where one log contains information for
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multiple keys. This would probably improve performance too due to
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caching, although in some cases git-annex would have to process extra
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information to get to the info about the key it wants, which hurts
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performance. The disk usage change of this method has not yet been
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quantified.
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* Another reason to do it would be improving git-annex to use vector clocks,
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instead of its current assumption that client's clocks are close enough to
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accurate. This would presumably change the contents of the files.
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* While not a sufficient reason on its own, the best practices for file
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formats in the git-annex branch has evolved over time, and there are some
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files that have unusual formats for historical reasons. Other files have
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modern formats, but their parsers have to cope with old versions that
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have other formats. A hard transition would provide an opportunity to
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clean up a lot of that.
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## living on the edge
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Rather than a hard transition, git-annex could add a v6 mode
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that could be optionally enabled when initing a repo for the first time.
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Users who know they need that mode could then turn it one, and get the
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benefits, while everyone else avoids a transition that doesn't benefit them
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much.
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There could even be multiple modes, with different tradeoffs depending on
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how the repo will be used, its size, etc. Of course that adds complexity.
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But the main problem with this idea is, how to avoid the foot shooting
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result of merging repo A(v5) into repo B(v6)? This seems like it would be
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all to easy for a user to do.
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As far as git-annex branch changes go, it might be possible for git-annex
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to paper over the problem by handling both versions in the merged git-annex
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branch, as discussed earlier. But for .git/annex/objects/ changes, there
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does not seem to be a reasonable thing for git-annex to do. When it's
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receiving an object into a mixed v5 and v6 repo, it can't know which
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location that repo expects the object file to be located in. Different
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files in the repo might point to the same object in different locations!
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Total mess. Must avoid this.
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Currently, annex.version is a per-local-repo setting. git-annex can't tell
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if two repos that it's merging have different annex.version's.
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It would be possible to add a git-annex:version file, which would work for
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git-annex branch merging. Ie, `git-annex merge` could detect if different
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git-annex branches have different versions, and refuse to merge them (or
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upgrade the old one before merging it).
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Also, that file could be used by git-annex, to automatically set
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annex.version when auto-initing a clone of a repo that was initted with
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a newer than default version.
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But git-anex:version won't prevent merging B/master into A's master.
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That merge can be done by git; nothing in git-annex can prevent it.
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What we could do is have a .annex-version flag file in the root of the
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repo. Then git merge would at least have a merge conflict. Note that this
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means inflicting the file on all git-annex repos, even ones used by people
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with no intention of living on the edge. And, it would take quite a while
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until all such repos get updated to contain such a file.
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Or, we could just document that if you initialize a repo with experimental
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annex.version, you're living on the edge and you can screw up your repo
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by merging with a repo from an old version.
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git-annex fsck could also fix up any broken links that do result from the
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inevitable cases where users ignore the docs.
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## version numbers vs configuration
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A particular annex.version like 5 encompasses a number of somewhat distinct
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things
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* git-annex branch layout
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* .git/annex/objects/ layout
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* other git stuff (like eg, the name of the HEAD branch in direct mode)
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If the user is specifying at `git annex init` time some nonstandard things
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they want to make the default meet their use case better, that is more
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a matter of configuration than of picking a version.
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For example, we could say that the user is opting out of the second-level
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object hash directories. Or we could say the user is choosing to use v6,
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which is like v5 except with different object hash directory structure.
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git annex init --config annex.objects.hashdirectories 1
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--config annex.objects.hashlower true
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git annex init --version 6
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The former would be more flexible. The latter is simpler.
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The former also lets the user chose *no* hash directories, or
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choose 2 levels of hash directories while using the (v5 default) mixed
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case hashing.
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## concrete design
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Make git-annex:difference.log be used by newer git-annex versions than v5,
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and by nonstandard configurations.
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The file contents will be "timestamp uuid [value, ..]", where value is a
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serialized data type that describes divergence from v5 (since v5 and older
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don't have the git-annex:difference.log file).
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So, for example, "[Version 6]" could indicate that v6 is being used. Or,
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"[ObjectHashLower True, ObjectHashDirectories 1, BranchHashDirectories 1]"
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indicate a nonstandard configuration on top of v5 (this might turn out to
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be identical to v6; just make the compare equal and no problem).
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git-annex merge would check if it's merging in a git-annex:difference.log from
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another repo that doesn't match the git-annex:difference.log of the local repo,
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and abort. git-annex sync (and the assistant) would check the same, but
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before merging master branches either, to avoid a bad merge there.
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The git-annex:difference.log of a local repo could be changed by an upgrade
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or some sort of transition. When this happens, the new value is written
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for the uuid of the local repo. git-annex merge would then refuse to merge
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with remote repos until they were also transitioned.
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(There's perhaps some overlap here with the existing
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git-annex:transitions.log, however the current transitions involve
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forgetting history/dead remotes and so can be done repeatedly on a
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repository. Also, the current transitions can be performed on remote
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branches before merging them in; that wouldn't work well for version
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changes since those require other changes in the remote repo.)
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Not covered:
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* git-merge of other branches, such as master (can be fixed by `git annex
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fix` or `fsck`)
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* Old versions of git-annex will ignore the version file of course,
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and so merging such repos using them can result in pain.
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