git-annex/doc/design/v6.mdwn

This page's purpose is to collect and explore plans for a future
annex.version 6.

There are two major possible changes that could go in v6 or a later
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 v6 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(v6)? 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 v6 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 v6,
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.