git-annex allows managing files with git, without checking the file contents into git. This is useful when dealing with files larger than git can currently easily handle, whether due to limitations in memory, checksumming time, or disk space (only one copy need be stored of an annexed file). Even without file content tracking, being able to manage file metadata with git, move files around and delete files with versioned directory trees, and use branches and distributed clone, are all very handy reasons to use git. And annexed files can co-exist in the same git repository with regularly versioned files, which is convenient for maintaining code, Makefiles, etc that are associated with annexed files but that benefit from full revision control. Enough broad picture, here's how it actually looks: * `git annex --add $file` moves the file into `.git/annex/`, and replaces it with a symlink pointing at the annexed file, and then calls `git add` to version the *symlink*. (If the file has already been annexed, it does nothing.) * You can move the symlink around, copy it, delete it, etc, and commit changes as desired using git. Reading the symlink will always get you the annexed file content, or the link may be broken if the content is not currently available. * If you use normal git push/pull commands, the annexed file contents won't be sent, but the symlinks will be. So different clones of a repository can have different sets of annexed files available. * `git annex --push $repository` pushes *all* annexed files to the specified repository. * `git annex --pull $repository` pulls *all* annexed files from the specified repository. * `git annex --want $file` indicates that you want access to a file's content, without immediatly transferring it. * `git annex --get $file` is used to transfer a specified file, and/or files previously indicated with --want. If a configured repository has it, or it is available from other key/value storage, it will be immediatly downloaded. * `git annex --drop $file` indicates that you no longer want the file's content to be available in this repository. * `git annex $file` is a shorthand for either --add or --get. If the file is already known, it does --get, otherwise it does --add. ## copies git-annex can be configured to try to keep N copies of a file's content available across all repositories. By default, N is 1 (configured by annex.numcopies). `git annex --drop` attempts to communicate with all other configured repositories, to check that N copies of the file exist. If enough repositories cannot be contacted, it will retain the file content. You can later use `git annex --drop --retry` to retry pending drops. Or you can use `git annex --drop --force $file` to force dropping of file content. For example, consider three repositories: Server, Laptop, and USB. Both Server and USB have a copy of a file, and N=1. If on Laptop, you `git annex --get $file`, this will transfer it from either Server or USB (depending on which is available), and there are now 3 copies of the file. Suppose you want to free up space on laptop again, and you --drop the file there. If USB is connected, or Server can be contacted, git-annex can check that it still has a copy of the file, and the content is removed from Laptop. But if USB is currently disconnected, and Server also cannot be contacted, it can't check that and will retain the file content. With N=2, in order to drop the file content from Laptop, it would need access to both USB and Server. Note that different repositories can be configured with different values of N. So just because Laptop has N=2, this does not prevent the number of copies falling to 1, when USB and Server have N=1, and of they have the only copies of a file. ## the .git-annex directory The `.git-annex` directory at the top of the repository, is used to store git-annex information that should be propigated between repositories. Data is stored here in files that are arranged to avoid conflicts in most cases. A conflict could occur if a file with the same name but different content was added to multiple clones. ## key/value storage git-annex uses a key/value abstraction layer to allow files contents to be stored in different ways. In theory, any key/value storage system could be used to store the file contents, and git-annex would then retrieve them as needed and put them in `.git/annex/`. When a file is annexed, a key is generated from its content and/or metadata. This key can later be used to retrieve the file's content (its value). This key generation must be stable for a given file content, name, and size. The mapping from filename to its key is stored in the .git-annex directory, in a file named `$filename.$backend` Multiple pluggable backends are supported, and more than one can be used to store different files' contents in a given repository. * `file` -- This backend stores the file's content in `.git/annex/`, and assumes that any file with the same basename has the same content. So with this backend, files can be moved around, but should never be added to or changed. This is the default, and the least expensive backend. * `sha1sum` -- This backend stores the file's content in `.git/annex/`, with a name based on its sha1 checksum. This backend allows modifications of files to be tracked. Its need to generate checksums can make it slow for large files. * `url` -- This backend downloads the file's content from an external URL. ## location tracking git-annex keeps track of on which repository it last saw a file's content. This can be useful when using it for archiving with offline storage. When you indicate you --want a file, git-annex will tell you which repositories have the file's content. Location tracking information is stored in `.git-annex/$filename.log`. Repositories record their name and the date when they --get or --drop a file's content. (Git is configured to use a union merge for this file, so the lines may be in arbitrary order, but it will never conflict.) ## configuration * `annex.numcopies` -- number of copies of files to keep * `annex.backend` -- name of the default key/value backend to use to store new files * `annex.name` -- allows specifying a unique name for this repository. If not specified, the name is derived from its directory's location and the hostname. When a repository is on removable media it is useful to give it a more stable name. Typically the name of a repository is the same name configured as a git remote to allow pulling from that repository. * `remote..annex-cost` -- When determining which repository to transfer annexed files from or to, ones with lower costs are preferred. The default cost is 50. Note that other factors may be configured when pushing files to repositories, in particular, whether the repository is on a filesystem with sufficient free space. ## issues ### symlinks If the symlink to annexed content is relative, moving it to a subdir will break it. But it it's absolute, moving the git repo (or mounting its drive elsewhere) will break it. Either: * Use relative links and need `git annex mv` to move (or post-commit hook that caches moves and updates links). * Use absolute links and need `git annex fixlinks` when location changes; note that would also mean that git would see the symlink targets changed and want to commit the change. ### free space determination Need a way to tell how much free space is available on the disk containing a given repository. The repository may be remote, so ssh may need to be used. Similarly, need a way to tell the size of a file before downloading it from remote, to check local disk space. ### auto-drop files on rm When git-rm removed a file, it should get dropped too. Of course, it may not be dropped right away, depending on number of copies available.