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= largefiles - manage large binary files =
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This extension is based off of Greg Ward's bfiles extension which can be found
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at http://mercurial.selenic.com/wiki/BfilesExtension.
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== The largefile store ==
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largefile stores are, in the typical use case, centralized servers that have
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every past revision of a given binary file. Each largefile is identified by
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its sha1 hash, and all interactions with the store take one of the following
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forms.
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-Download a bfile with this hash
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-Upload a bfile with this hash
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-Check if the store has a bfile with this hash
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largefiles stores can take one of two forms:
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-Directories on a network file share
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-Mercurial wireproto servers, either via ssh or http (hgweb)
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== The Local Repository ==
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The local repository has a largefile cache in .hg/largefiles which holds a
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subset of the largefiles needed. On a clone only the largefiles at tip are
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downloaded. When largefiles are downloaded from the central store, a copy is
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saved in this store.
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== The Global Cache ==
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largefiles in a local repository cache are hardlinked to files in the global
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cache. Before a file is downloaded we check if it is in the global cache.
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== Implementation Details ==
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Each largefile has a standin which is in .hglf. The standin is tracked by
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Mercurial. The standin contains the SHA1 hash of the largefile. When a
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largefile is added/removed/copied/renamed/etc the same operation is applied to
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the standin. Thus the history of the standin is the history of the largefile.
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For performance reasons, the contents of a standin are only updated before a
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commit. Standins are added/removed/copied/renamed from add/remove/copy/rename
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Mercurial commands but their contents will not be updated. The contents of a
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standin will always be the hash of the largefile as of the last commit. To
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support some commands (revert) some standins are temporarily updated but will
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be changed back after the command is finished.
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A Mercurial dirstate object tracks the state of the largefiles. The dirstate
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uses the last modified time and current size to detect if a file has changed
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(without reading the entire contents of the file).
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