git-annex/git-annex.mdwn

git-annex allows managing files with git, without checking the file
contents into git. This is useful when dealing with files larger than git
can currently easily handle, whether due to limitations in memory,
checksumming time, or disk space (only one copy need be stored of an
annexed file). 

Even without file content tracking, being able to manage file metadata with
git, move files around and delete files with versioned directory trees, and use
branches and distributed clone, are all very handy reasons to use git. And
annexed files can co-exist in the same git repository with regularly versioned
files, which is convenient for maintaining code, Makefiles, etc that are
associated with annexed files but that benefit from full revision control. 

Enough broad picture, here's how it actually looks:

* `git annex --add $file` moves the file into `.git/annex/`, and replaces
  it with a symlink pointing at the annexed file, and then calls `git add`
  to version the *symlink*. (If the file has already been annexed, it does
  nothing.)
* You can move the symlink around, copy it, delete it, etc, and commit changes
  as desired using git. Reading the symlink will always get you the annexed
  file content, or the link may be broken if the content is not currently
  available.
* If you use normal git push/pull commands, the annexed file contents
  won't be sent, but the symlinks will be. So different clones of a repository
  can have different sets of annexed files available.
* `git annex --push $repository` pushes *all* annexed files to the specified
  repository.
* `git annex --pull $repository` pulls *all* annexed files from the specified
  repository.
* `git annex --want $file` indicates that you want access to a file's
  content, without immediatly transferring it.
* `git annex --get $file` is used to transfer a specified file, and/or
  files previously indicated with --want. If a configured repository has it,
  or it is available from other key/value storage, it will be immediatly
  downloaded.
* `git annex --drop $file` indicates that you no longer want the file's
  content to be available in this repository.
* `git annex --unannex $file` undoes a `git annex --add`.
* `git annex $file` is a shorthand for either --add or --get. If the file
  is already known, it does --get, otherwise it does --add.

## copies

git-annex can be configured to try to keep N copies of a file's content
available across all repositories. By default, N is 1 (configured by
annex.numcopies).

`git annex --drop` attempts to communicate with all other configured
repositories, to check that N copies of the file exist. If enough
repositories cannot be contacted, it will retain the file content.
You can later use `git annex --drop --retry` to retry pending drops.
Or you can use `git annex --drop --force $file` to force dropping of
file content.

For example, consider three repositories: Server, Laptop, and USB. Both Server
and USB have a copy of a file, and N=1. If on Laptop, you `git annex --get
$file`, this will transfer it from either Server or USB (depending on which
is available), and there are now 3 copies of the file.

Suppose you want to free up space on laptop again, and you --drop the file
there. If USB is connected, or Server can be contacted, git-annex can check
that it still has a copy of the file, and the content is removed from
Laptop. But if USB is currently disconnected, and Server also cannot be
contacted, it can't check that and will retain the file content.

With N=2, in order to drop the file content from Laptop, it would need access
to both USB and Server.

Note that different repositories can be configured with different values of
N. So just because Laptop has N=2, this does not prevent the number of
copies falling to 1, when USB and Server have N=1, and of they have the
only copies of a file.

## the .git-annex directory

The `.git-annex` directory at the top of the repository, is used to store
git-annex information that should be propigated between repositories.

Data is stored here in files that are arranged to avoid conflicts in most
cases. A conflict could occur if a file with the same name but different
content was added to multiple repositories.

## key/value storage

git-annex uses a key/value abstraction layer to allow files contents to be
stored in different ways. In theory, any key/value storage system could be
used to store the file contents, and git-annex would then retrieve them
as needed and put them in `.git/annex/`.

When a file is annexed, a key is generated from its content and/or metadata.
This key can later be used to retrieve the file's content (its value). This
key generation must be stable for a given file content, name, and size.

The mapping from filename to its key is stored in the .git-annex directory,
in a file named `$filename.$backend`

Multiple pluggable backends are supported, and more than one can be used
to store different files' contents in a given repository.

* `file` -- This backend stores the file's content in
  `.git/annex/`, and assumes that any file with the same basename
  has the same content. So with this backend, files can be moved around,
  but should never be added to or changed. This is the default, and
  the least expensive backend.
* `sha1sum` -- This backend stores the file's content in
  `.git/annex/`, with a name based on its sha1 checksum. This backend allows
  modifications of files to be tracked. Its need to generate checksums
  can make it slow for large files.
* `url` -- This backend downloads the file's content from an external URL.

## location tracking

git-annex keeps track of on which repository it last saw a file's content.
This can be useful when using it for archiving with offline storage. When
you indicate you --want a file, git-annex will tell you which repositories
have the file's content.

Location tracking information is stored in `.git-annex/$filename.log`.
Repositories record their UUID and the date when they --get or --drop 
a file's content. (Git is configured to use a union merge for this file,
so the lines may be in arbitrary order, but it will never conflict.)

The optional file `.git-annex/uuid.map` can be created to add a description
to a UUID. If git-annex needs a file from a repository and it cannot find
the repository amoung the remotes, it will use the description from this
file when asking for the repository to be made available. The file format
is a UUID, a space, and the rest of the line is its description. For
example:

	UUID d3d2474c-d5c3-11df-80a9-002170d25c55 USB drive in red enclosure

## configuration

* `annex.uuid` -- a unique UUID for this repository
* `annex.numcopies` -- number of copies of files to keep (default: 1)
* `annex.backends` -- space-separated list of names of 
  the key/value backends to use. The first listed is used to store
  new files. (default: file, checksum, url)
* `remote.<name>.annex-cost` -- When determining which repository to
  transfer annexed files from or to, ones with lower costs are preferred.
  The default cost is 50. Note that other factors may be configured
  when pushing files to repositories, in particular, whether the repository
  is on a filesystem with sufficient free space.
* `remote.<name>.annex-uuid` -- git-annex caches UUIDs of remotes here

## issues

### symlinks

If the symlink to annexed content is relative, moving it to a subdir will
break it. But it it's absolute, moving the git repo (or mounting its drive
elsewhere) will break it. Either:

* Use relative links and need `git annex --mv` to move (or post-commit 
  hook that caches moves and updates links).
* Use absolute links and need `git annex fixlinks` when location changes;
  note that would also mean that git would see the symlink targets changed
  and want to commit the change. And, other clones of the repo would
  diverge and there would be conflicts on the symlink text. Ugh.

Hard links are not an option, because git would then happily commit the
file content. Amoung other reasons..

### free space determination

Need a way to tell how much free space is available on the disk containing
a given repository. The repository may be remote, so ssh may need to be
used.

Similarly, need a way to tell the size of a file before downloading it from
remote, to check local disk space.

### auto-drop files on rm

When git-rm removed a file, it should get dropped too. Of course, it may
not be dropped right away, depending on number of copies available.