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.
These backends can transfer file contents between configured git remotes. It's also possible to use special remotes, such as Amazon S3 with these backends.
WORM("Write Once, Read Many") This backend 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 uses a key based on a sha1 checksum. This backend allows modifications of files to be tracked. Its need to generate checksums can make it slower for large files.SHA512,SHA384,SHA256,SHA224-- Like SHA1, but larger checksums. Mostly useful for the very paranoid, or anyone who is researching checksum collisions and wants to annex their colliding data. ;)SHA1E,SHA512E, etc -- Variants that preserve filename extension as part of the key. Useful for archival tasks where the filename extension contains metadata that should be preserved.
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 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 annex.backend=SHA1
*.ogg annex.backend=SHA1