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-[[!meta title="Missing node or KeyError problems"]]
-
-Obnam prior to version 1.6.1, and [Larch][] prior to version
-1.20131130, had serious, but somewhat rare bugs that could result in
-an error that said "Missing node" or "KeyError". These bugs would
-cause a corruption in the backup repository by either causing a file
-to be deleted, when it shouldn't have been, or the reference count for
-a B-tree node to be wrong. As a result, Obnam would become unable to
-make a backup or, in some cases, to restore everything from a backup
-(some files could be restored).
-
-The corruption is not necessarily noticeable on all backup runs. It
-depends on the exact files that get modified during a backup
-generation. Worse, `obnam fsck` isn't able to fix the corruption,
-though it would find it.
-
-As a result, a corrupted repository may be unrecoverable. Sometimes
-you can recover at least some of the repository, with some loss of
-data:
-
-* If the corruption is only in a "per-client B-tree", recovery can
- happen by removing or renaming that B-tree, but that results in all
- backup generations for that client being lost. However, file content
- will remain in the repository, so a fresh full backup should be
- reasonably fast. The per-client B-tree is a directory that has a
- large number as its name. The directory needs to be removed manully,
- not using Obnam. Instead of removing it, you can also rename it.
-
-* If the corruption is in the `chunksums` or `chunklist` B-trees, you
- can delete or rename them both. The next backup run will create new
- ones. This will reduce the possibility of de-duplication, but
- otherwise everything should work.
-
-If the corruption is any other B-tree, no recovery is possible.
-
-When in doubt, avoid using a repository that has ever been used with
-Obnam prior to version 1.6.1, or Larch prior to version 1.20131130.
-Instead, start over with a new, empty repository.
-
-Lars apologises for this.
-
-[Larch]: http://liw.fi/larch/