git-annex tries to ensure that the configured number of [[copies]] of your
data always exist, and leaves it up to you to use commands like `git annex
get` and `git annex drop` to move the content to the repositories you want
to contain it. But sometimes, it can be good to have more fine-grained
control over which repositories prefer to have which content. Configuring
this allows `git annex get --auto`, `git annex drop --auto`, etc to do
smarter things.

Currently, preferred content settings can only be edited using `git
annex vicfg`. Each repository can have its own settings, and other
repositories may also try to honor those settings. So there's no local
`.git/config` setting it.

The idea is that you write an expression that files are matched against.
If a file matches, it's preferred to have its content stored in the
repository. If it doesn't, it's preferred to drop its content from
the repository (if there are enough copies elsewhere).

The expressions are very similar to the file matching options documented
on the [[git-annex]] man page. At the command line, you can use those
options in commands like this:

	git annex get --include='*.mp3' --and -'(' --not --in=archive -')'

The equivilant preferred content expression looks like this:

	include=*.mp3 and (not in=archive)

So, just remove the dashes, basically.

## file matching

Note that while --include and --exclude match files relative to the current
directory, preferred content expressions always match files relative to the
top of the git repository. Perhaps you put files into `archive` directories
when you're done with them. Then you could configure your laptop to prefer
to not retain those files, like this:

	exclude=*/archive/*

## standard expressions

git-annex comes with some standard preferred content expressions, that can
be used with repositories that are in some pre-defined groups. To make a
repository use one of these, just set its preferred content expression
to "standard", and put it in one of these groups:

### client

All content is preferred, unless it's in a "archive" directory.

`exclude=*/archive/*`

### transfer

Use for repositories that are used to transfer data between other
repositories, but do not need to retain data themselves. For
example, a repository on a server, or in the cloud, or a small
USB drive used in a sneakernet.

The preferred content expression for these causes them to get and retain
data until all clients have a copy.

`not inallgroup=client and exclude=*/archive/*`

### archive

All content is preferred, unless it's already been archived somewhere else.

`not copies=archive:1`

Note that if you want to archive multiple copies (not a bad idea!),
you should instead configure all your archive repositories with a
version of the above preferred content expression with a larger
number of copies.

### backup

All content is preferred.