109 lines
3.6 KiB
Markdown
109 lines
3.6 KiB
Markdown
This page tries to regroup a set of Really Bad Ideas people had with
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git-annex in the past that can lead to catastrophic data loss, abusive
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disk usage, improper swearing and other unfortunate experiences.
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This could also be called the "git annex worst practices", but is
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different than [[not|what git annex is not]] in that it covers normal
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use cases of git-annex, just implemented in the wrong way. Hopefully,
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git-annex should make it as hard as possible to do those things, but
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sometimes, you just can't help it, people figure out the worst
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possible ways of doing things.
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[[!toc]]
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.git/annex symlink
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==================
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Antipattern
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-----------
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Symlinking the `.git/annex` symlink directory, in the hope of saving
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disk space, is a horrible idea. The general antipattern is:
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git clone repoA repoB
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mv repoB/.git/annex repoB/.git/annex.bak
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ln -s repoA/.git/annex repoB/.git/annex
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This is bad because git-annex will believe it has two copy of the
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files and then would let you drop the single copy, therefore leading
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to data loss.
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Proper pattern
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--------------
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The proper way of doing this is through git-annex's hardlink support,
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by cloning the repository with the `--shared` option:
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git clone --shared repoA repoB
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This will setup repoB as an "untrusted" repository and use hardlinks
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to copy files between the two repos, using space only once. This
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works, of course, only on filesystems that support hardlinks, but
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that's usually the case for filesystems that support symlinks.
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Real world cases
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----------------
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* [[forum/share_.git__47__annex__47__objects_across_multiple_repositories_on_one_machine/]]
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* at least one IRC discussion
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Fixes
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-----
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Probably no way to fix this in git-annex - if users want to shoot
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themselves in the foot by messing with the backend, there's not much
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we can do to change that in this case.
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using reinit with an existing uuid without fsck
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===============================================
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To quote the [[git-annex-reinit]] manpage:
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> Normally, initializing a repository generates a new, unique
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> identifier (UUID) for that repository. Occasionally it may be useful
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> to reuse a UUID -- for example, if a repository got deleted, and
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> you're setting it back up.
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Anti-pattern
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------------
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[[git-annex-reinit]] can be used to reuse UUIDs for deleted
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repositories. But what happens if you reuse the UUID of an *existing*
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repository, or a repository that hasn't been properly emptied before
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being declared dead? This can lead to data loss because, in that case,
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git-annex may think some files are still present in the revived
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repository (while they may not actually be).
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Proper pattern
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--------------
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The proper way of using reinit is to make sure you run
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[[git-annex-fsck]] (optionally with `--fast` to save time) on the
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revived repo right after running reinit. This will ensure that at
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least the location log will be updated, and git-annex will notice if
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files are missing.
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Real world cases
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----------------
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* [[bugs/remotes_disappeared]]
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Fixes
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-----
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An improvement to git-annex here would be to allow
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[[todo/reinit_should_work_without_arguments|reinit to work without arguments]]
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to at least not encourage UUID reuse. reinit could also recommend
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running fsck explicitely. It could even trigger an fsck directly.
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The [[git-annex-reinit]] manpage has always suggested running `fsck`,
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but the wording has been changed on 2017-01-17.
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Other cases
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===========
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Feel free to add your lessons in catastrophe here! It's educational
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and fun, and will improve git-annex for everyone.
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PS: should this be a toplevel page instead of being drowned in the
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[[tips]] section? Where should it be linked to? -- [[anarcat]]
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