git-annex uses a key-value abstraction layer to allow file contents to be
stored in different ways. In theory, any key-value storage system could be
used to store file contents.

When a file is annexed, a key is generated from its content and/or metadata.
The file checked into git symlinks to the key. This key can later be used
to retrieve the file's content (its value).

Multiple pluggable backends are supported, and a single repository
can use different backends for different files.

* `WORM` ("Write Once, Read Many") This backend stores the file's content
  only in `.git/annex/objects/`, and assumes that any file with the same
  basename, size, and modification time 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.
* `SHA1` -- This backend stores the file's content in
  `.git/annex/objects/`, 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 slower for large files.
  for use.
* `URL` -- This backend downloads the file's content from an external URL.

The `annex.backends` git-config setting can be used to list the backends
git-annex should use. The first one listed will be used by default when
new files are added.

For finer control of what backend is used when adding different types of
files, the `.gitattributes` file can be used. The `git-annex-backend`
attribute can be set to the name of the backend to use for matching files.

For example, to use the SHA1 backend for sound files, which tend to be
smallish and might be modified over time, you could set in
`.gitattributes`:

	*.mp3 git-annex-backend=SHA1
	*.ogg git-annex-backend=SHA1