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diff --git a/doc/design/new_repo_versions.mdwn b/doc/design/new_repo_versions.mdwn new file mode 100644 index 000000000..a42b52b1a --- /dev/null +++ b/doc/design/new_repo_versions.mdwn @@ -0,0 +1,233 @@ +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. + |