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The unit for compressed storage in bzr is a *revfile*, whose design
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was suggested by Matt Mackall.
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This document describes version 1 of the file, and has some notes on
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what might be done in version 2.
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Compressed storage is a tradeoff between several goals:
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* Reasonably compact storage of long histories.
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* Robustness and simplicity.
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* Fast extraction of versions and addition of new versions (preferably
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without rewriting the whole file, or reading the whole history.)
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* Fast and precise annotations.
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* Storage of files of at least a few hundred MB.
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* Lossless in useful ways: we can extract a series of texts and write
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them back out without losing any information.
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revfiles store the history of a single logical file, which is
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identified in bzr by its file-id. In this sense they are similar to
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an RCS or CVS ``,v`` file or an SCCS sfile.
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Each state of the file is called a *text*.
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Renaming, adding and deleting this file is handled at a higher level
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by the inventory system, and is outside the scope of the revfile. The
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revfile name is typically based on the file id which is itself
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typically based on the name the file had when it was first added. But
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this is purely cosmetic.
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For example a file now called ``frob.c`` may have the id
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``frobber.c-12873`` because it was originally called
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``frobber.c``. Its texts are kept in the revfile
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``.bzr/revfiles/frobber.c-12873.revs``.
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When the file is deleted from the inventory the revfile does not
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change. It's just not used in reproducing trees from that point
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The revfile does not record the date when the text was added, a commit
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message, properties, or any other metadata. That is handled in the
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higher-level revision history.
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Inventories and other metadata files that vary from one version to the
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next can themselves be stored in revfiles.
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revfiles store files as simple byte streams, with no consideration of
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translating character sets, line endings, or keywords. Those are also
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handled at a higher level. However, the revfile may make use of
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knowledge that a file is line-based in generating a diff.
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(The Python builtin difflib is too slow when generating a purely
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byte-by-byte delta so we always make a line-by-line diff; when this
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is fixed it may be feasible to use line-by-line diffs for all
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Files whose text does not change from one revision to the next are
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stored as just a single text in the revfile. This can happen even if
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the file was renamed or other properties were changed in the
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The revfile is held on disk as two files: an *index* and a *data*
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file. The index file is short and always read completely into memory;
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the data file is much longer and only the relevant bits of it,
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identified by the index file, need to be read.
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This design is similar to that of Netscape `mail summary files`_, in
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that there is a small index which can always be read into memory and
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that quickly identifies where to look in the main file. They differ
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in many other ways though, most particularly that the index is not
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just a cache but holds precious data in its own right.
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.. _`mail summary files`: http://www.jwz.org/doc/mailsum.html
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This is meant to scale to hold 100,000 revisions of a single file, by
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which time the index file will be ~4.8MB and a bit big to read
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Some of the reserved fields could be used to implement a (semi?)
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balanced tree indexed by SHA1 so we can much more efficiently find the
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index associated with a particular hash. For 100,000 revs we would be
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able to find it in about 17 random reads, which is not too bad. On
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the other hand that would compromise the append-only indexing, and
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100,000 revs is a fairly extreme case.
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This performs pretty well except when trying to calculate deltas of
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really large files. For that the main thing would be to plug in
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something faster than difflib, which is after all pure Python.
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Another approach is to just store the gzipped full text of big files,
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though perhaps that's too perverse?
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In the current version, texts are identified by their SHA-1.
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Because the basis of a delta does not need to be the text's logical
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predecessor, we can adjust the deltas to avoid ever needing to apply
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too many deltas to reproduce a particular file.
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The revfile module can be invoked as a program to give low-level
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access for data recovery, debugging, etc.
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* revfiles use unsigned 32-bit integers both in diffs and the index.
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This should be more than enough for any reasonable source file but
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perhaps not enough for large binaries that are frequently committed.
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Perhaps for those files there should be an option to continue to use
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the text-store. There is unlikely to be any benefit in holding
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deltas between them, and deltas will anyhow be hard to calculate.
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* The append-only design does not allow for destroying committed data,
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as when confidential information is accidentally added. That could
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be fixed by creating the fixed repository as a separate branch, into
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which only the preserved revisions are exported.
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* Should we also store full-texts as a transitional step?
added
removed
doc/revfile.txt
