52 lines
2.6 KiB
Markdown
52 lines
2.6 KiB
Markdown
When a file is annexed, a key is generated from its content and/or metadata.
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The file checked into git symlinks to the key. This key can later be used
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to retrieve the file's content (its value).
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Multiple pluggable key-value backends are supported, and a single repository
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can use different ones for different files.
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* `SHA256E` -- The default backend for new files, combines a 256 bit SHA-2
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hash of the file's content with the file's extension. This allows
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verifying that the file content is right, and can avoid duplicates of
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files with the same content. Its need to generate checksums
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can make it slower for large files.
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* `SHA256` -- SHA-2 hash that does not include the file extension in the
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key, which can lead to better deduplication but can confuse some programs.
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* `SHA512`, `SHA512E` -- Best SHA-2 hash, for the very paranoid.
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* `SHA384`, `SHA384E`, `SHA224`, `SHA224E` -- SHA-2 hashes for
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people who like unusual sizes.
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* `SHA3_512`, `SHA3_512E`, `SHA3_384`, `SHA3_384E`, `SHA3_256`, `SHA3_256E`, `SHA3_224`, `SHA3_224E`
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-- SHA-3 hashes, for bleeding edge fun.
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* `SKEIN512`, `SKEIN512E`, `SKEIN256`, `SKEIN256E`
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-- [Skein hash](http://en.wikipedia.org/wiki/Skein_hash),
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a well-regarded SHA3 hash competition finalist.
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* `SHA1`, `SHA1E`, `MD5`, `MD5E` -- Smaller hashes than `SHA256`
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for those who want a checksum but are not concerned about security.
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* `WORM` ("Write Once, Read Many") -- This assumes that any file with
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the same filename, size, and modification time has the same content.
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This is the least expensive backend, recommended for really large
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files or slow systems.
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* `URL` -- This is a key that is generated from the url to a file.
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It's generated when using eg, `git annex addurl --fast`, when the file
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content is not available for hashing.
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Note that the various 512 and 384 length hashes result in long paths,
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which are known to not work on Windows. If interoperability on Windows is a
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concern, avoid those.
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The `annex.backends` git-config setting can be used to list the backends
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git-annex should use. The first one listed will be used by default when
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new files are added.
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For finer control of what backend is used when adding different types of
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files, the `.gitattributes` file can be used. The `annex.backend`
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attribute can be set to the name of the backend to use for matching files.
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For example, to use the SHA256E backend for sound files, which tend to be
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smallish and might be modified or copied over time,
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while using the WORM backend for everything else, you could set
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in `.gitattributes`:
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* annex.backend=WORM
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*.mp3 annex.backend=SHA256E
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*.ogg annex.backend=SHA256E
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