40aab719df
in= was problimatic in two ways. First, it referred to a remote by name, but preferred content expressions can be evaluated elsewhere, where that remote doesn't exist, or a different remote has the same name. This name lookup code could error out at runtime. Secondly, in= seemed pretty useless. in=here did not cause content to be gotten, but it did let present content be dropped. present is more useful, although "not present" is unstable and should be avoided.
128 lines
5.1 KiB
Markdown
128 lines
5.1 KiB
Markdown
git-annex tries to ensure that the configured number of [[copies]] of your
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data always exist, and leaves it up to you to use commands like `git annex
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get` and `git annex drop` to move the content to the repositories you want
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to contain it. But sometimes, it can be good to have more fine-grained
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control over which repositories prefer to have which content. Configuring
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this allows `git annex get --auto`, `git annex drop --auto`, etc to do
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smarter things.
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Currently, preferred content settings can only be edited using `git
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annex vicfg`. Each repository can have its own settings, and other
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repositories may also try to honor those settings. So there's no local
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`.git/config` setting it.
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The idea is that you write an expression that files are matched against.
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If a file matches, it's preferred to have its content stored in the
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repository. If it doesn't, it's preferred to drop its content from
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the repository (if there are enough copies elsewhere).
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The expressions are very similar to the file matching options documented
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on the [[git-annex]] man page. At the command line, you can use those
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options in commands like this:
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git annex get --include='*.mp3' --and -'(' --not --largerthan=100mb -')'
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The equivilant preferred content expression looks like this:
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include=*.mp3 and (not largerthan=100mb)
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So, just remove the dashes, basically. However, there are some differences
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from the command line options to keep in mind:
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### difference: file matching
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While --include and --exclude match files relative to the current
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directory, preferred content expressions always match files relative to the
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top of the git repository. Perhaps you put files into `archive` directories
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when you're done with them. Then you could configure your laptop to prefer
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to not retain those files, like this:
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exclude=*/archive/*
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### difference: no "in="
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Preferred content expressions have no direct equivilant to `--in`.
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Often, it's best to add repositories to groups, and match against
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the groups in a preferred content expression. So rather than
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`--in=usbdrive`, put all the USB drives into a "transfer" group,
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and use "copies=transfer:1"
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### difference: dropping
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To decide if content should be dropped, git-annex evaluates the preferred
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content expression under the assumption that the content has *already* been
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dropped. If the content would not be preferred then, the drop can be done.
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So, for example, `copies=2` in a preferred content expression lets
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content be dropped only when there are currently 3 copies of it, including
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the repo it's being dropped from. This is different than running `git annex
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drop --copies=2`, which will drop files that current have 2 copies.
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A wrinkle of this approach is how `in=` is handled. When deciding if
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content should be dropped, git-annex looks at the current status, not
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the status if the content would be dropped. So `in=here` means that
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any currently present content is preferred, which can be useful if you
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want manual control over content. Meanwhile `not (in=here)` should be
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avoided -- it will cause content that's not here to be preferred,
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but once the content arrives, it'll stop being preferred and will be
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dropped again!
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## difference: "present"
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There's a special "present" keyword you can use in a preferred content
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expression. This means that content is preferred if it's present,
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and not otherwise. This leaves it up to you to use git-annex manually
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to move content around. You can use this to avoid preferred content
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settings from affecting a subdirectory. For example:
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auto/* or (include=ad-hoc/* and present)
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Note that `not present` is a very bad thing to put in a preferred content
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expression. It'll make it prefer to get content that's not present, and
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drop content that is present! Don't go there..
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## standard expressions
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git-annex comes with some standard preferred content expressions, that can
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be used with repositories that are in some pre-defined groups. To make a
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repository use one of these, just set its preferred content expression
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to "standard", and put it in one of these groups:
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### client
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All content is preferred, unless it's in a "archive" directory.
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`exclude=*/archive/*`
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### transfer
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Use for repositories that are used to transfer data between other
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repositories, but do not need to retain data themselves. For
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example, a repository on a server, or in the cloud, or a small
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USB drive used in a sneakernet.
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The preferred content expression for these causes them to get and retain
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data until all clients have a copy.
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`not (inallgroup=client and copies=client:2) and exclude=*/archive/*`
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The "copies=client:2" part of the above handles the case where
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there is only one client repository. It makes a transfer repository
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speculatively prefer content in this case, even though it as of yet
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has nowhere to transfer it to. Presumably, another client repository
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will be added later.
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### archive
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All content is preferred, unless it's already been archived somewhere else.
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`not copies=archive:1`
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Note that if you want to archive multiple copies (not a bad idea!),
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you should instead configure all your archive repositories with a
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version of the above preferred content expression with a larger
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number of copies.
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### backup
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All content is preferred.
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