git-annex/doc/preferred_content.mdwn

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 --largerthan=100mb -')'

The equivilant preferred content expression looks like this:

	include=*.mp3 and (not largerthan=100mb)

So, just remove the dashes, basically. However, there are some differences
from the command line options to keep in mind:

### difference: file matching

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/*

### difference: no "in="

Preferred content expressions have no direct equivilant to `--in`.

Often, it's best to add repositories to groups, and match against
the groups in a preferred content expression. So rather than
`--in=usbdrive`, put all the USB drives into a "transfer" group,
and use "copies=transfer:1"

### difference: dropping

To decide if content should be dropped, git-annex evaluates the preferred
content expression under the assumption that the content has *already* been
dropped. If the content would not be preferred then, the drop can be done.
So, for example, `copies=2` in a preferred content expression lets
content be dropped only when there are currently 3 copies of it, including
the repo it's being dropped from. This is different than running `git annex
drop --copies=2`, which will drop files that current have 2 copies.

A wrinkle of this approach is how `in=` is handled. When deciding if
content should be dropped, git-annex looks at the current status, not
the status if the content would be dropped. So `in=here` means that
any currently present content is preferred, which can be useful if you
want manual control over content. Meanwhile `not (in=here)` should be
avoided -- it will cause content that's not here to be preferred,
but once the content arrives, it'll stop being preferred and will be
dropped again!

## difference: "present"

There's a special "present" keyword you can use in a preferred content
expression. This means that content is preferred if it's present,
and not otherwise. This leaves it up to you to use git-annex manually
to move content around. You can use this to avoid preferred content
settings from affecting a subdirectory. For example:

	auto/* or (include=ad-hoc/* and present)

Note that `not present` is a very bad thing to put in a preferred content 
expression. It'll make it prefer to get content that's not present, and
drop content that is present! Don't go there..

## 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 copies=client:2) and exclude=*/archive/*`

The "copies=client:2" part of the above handles the case where
there is only one client repository. It makes a transfer repository
speculatively  prefer content in this case, even though it as of yet
has nowhere to transfer it to. Presumably, another client repository
will be added later.

### 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.