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+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 more than one can be used
+to store different files' contents in a given repository.
+
+* `WORM` ("Write Once, Read Many") This backend stores the file's content
+  only in `.git/annex/`, 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/`, 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.
+* `URL` -- This backend downloads the file's content from an external URL.