git-annex/doc/backends.mdwn
Joey Hess b6d46c212e git-annex (5.20140402) unstable; urgency=medium
* unannex, uninit: Avoid committing after every file is unannexed,
    for massive speedup.
  * --notify-finish switch will cause desktop notifications after each
    file upload/download/drop completes
    (using the dbus Desktop Notifications Specification)
  * --notify-start switch will show desktop notifications when each
    file upload/download starts.
  * webapp: Automatically install Nautilus integration scripts
    to get and drop files.
  * tahoe: Pass -d parameter before subcommand; putting it after
    the subcommand no longer works with tahoe-lafs version 1.10.
    (Thanks, Alberto Berti)
  * forget --drop-dead: Avoid removing the dead remote from the trust.log,
    so that if git remotes for it still exist anywhere, git annex info
    will still know it's dead and not show it.
  * git-annex-shell: Make configlist automatically initialize
    a remote git repository, as long as a git-annex branch has
    been pushed to it, to simplify setup of remote git repositories,
    including via gitolite.
  * add --include-dotfiles: New option, perhaps useful for backups.
  * Version 5.20140227 broke creation of glacier repositories,
    not including the datacenter and vault in their configuration.
    This bug is fixed, but glacier repositories set up with the broken
    version of git-annex need to have the datacenter and vault set
    in order to be usable. This can be done using git annex enableremote
    to add the missing settings. For details, see
    http://git-annex.branchable.com/bugs/problems_with_glacier/
  * Added required content configuration.
  * assistant: Improve ssh authorized keys line generated in local pairing
    or for a remote ssh server to set environment variables in an
    alternative way that works with the non-POSIX fish shell, as well
    as POSIX shells.

# imported from the archive
2014-04-02 21:42:53 +01:00

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Markdown

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.
* `SHA256E` -- The default backend for new files, combines a SHA256 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` -- Does not include the file extension in the key, which can
lead to better deduplication but can confuse some programs.
* `WORM` ("Write Once, Read Many") This assumes that any file with
the same basename, size, and modification time has the same content.
This is the least expensive backend, recommended for really large
files or slow systems.
* `SHA512`, `SHA512E` -- Best currently available hash, for the very paranoid.
* `SHA1`, `SHA1E` -- Smaller hash than `SHA256` for those who want a checksum
but are not concerned about security.
* `SHA384`, `SHA384E`, `SHA224`, `SHA224E` -- Hashes for people who like
unusual sizes.
* `SKEIN512`, `SKEIN256` -- [Skein hash](http://en.wikipedia.org/wiki/Skein_hash),
a well-regarded SHA3 hash competition finalist.
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 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