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 key-value backends are supported, and a single repository can use different ones for different files.

These are the recommended backends to use.

  • SHA256E -- The default backend for new files, combines a 256 bit SHA-2 hash of the file's content with the file's extension. This allows verifying that the file content is right, and can avoid duplicates of files with the same content. Its need to generate checksums can make it slower for large files.
  • SHA256 -- SHA-2 hash that does not include the file extension in the key, which can lead to better deduplication but can confuse some programs.
  • SHA512, SHA512E -- Best SHA-2 hash, for the very paranoid.
  • SHA384, SHA384E, SHA224, SHA224E -- SHA-2 hashes for people who like unusual sizes.
  • SHA3_512, SHA3_512E, SHA3_384, SHA3_384E, SHA3_256, SHA3_256E, SHA3_224, SHA3_224E -- SHA-3 hashes, for bleeding edge fun.
  • SKEIN512, SKEIN512E, SKEIN256, SKEIN256E -- Skein hash, a well-regarded SHA3 hash competition finalist.

The backends below do not guarantee cryptographically that the content of an annexed file remains unchanged.

  • SHA1, SHA1E, MD5, MD5E -- Smaller hashes than SHA256 for those who want a checksum but are not concerned about security.
  • WORM ("Write Once, Read Many") -- This assumes that any file with the same filename, size, and modification time has the same content. This is the least expensive backend, recommended for really large files or slow systems.
  • URL -- This is a key that is generated from the url to a file. It's generated when using eg, git annex addurl --fast, when the file content is not available for hashing.

If you want to be able to prove that you're working with the same file contents that were checked into a repository earlier, you should avoid using the non-cryptographically-secure backends, and will need to use signed git commits. See ?using signed git commits for details.

Note that the various 512 and 384 length hashes result in long paths, which are known to not work on Windows. If interoperability on Windows is a concern, avoid those.

The annex.backends git-config setting can be used to list the backends git-annex should use when adding new files. The first one listed will be used.

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

For example, to use the SHA256E backend for sound files, which tend to be smallish and might be modified or copied over time, while using the WORM backend for everything else, you could set in .gitattributes:

* annex.backend=WORM
*.mp3 annex.backend=SHA256E
*.ogg annex.backend=SHA256E