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