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RFC: Delta indexing and composition

From: Branko Čibej <brane_at_xbc.nu>
Date: 2001-07-27 06:11:04 CEST

Here's the "more on that later" I promised.

Now that I've safely postponed the delta combiner, I'd like to share my
ideas for improving the deltification code in the filesystem.

The scheme we have in place now works (kudos to Mike and Karl here!),
but wastes both time and space, and is probably a mild disaster for
non-sequential access to large files or old versions. The reasons for
this are obvious:

   1. It reads/resurrects the whole text from the beginning up to the
      end of the interesting region
   2. It keeps applying all the deltas from the version to the fulltext
      every time an old version is accessed, throwing away the result.

My suggested solution to these two problems is based on recognizing
exactly what a delta window is:

    A delta window defines a contiguous region of text A. It may depend
    on a contiguous region of text B, but is independent of any other
    delta window in the delta representation of A.

Given this definition, two things become obvious:

   1. To reconstruct the region defined by window N, we only have to
      read that window.
   2. Different windows within a delta may depend on different source texts.

The first item implies that we can solve the first problem my indexing
the windows within a delta, and accessing them directly. But we knew
that already.

The second item implies that every time we reconstruct a region of text,
we can replace its defining delta window with a single diff from the
fulltext, eliminating the intermediate reconstruction steps the next
time this region is accessed -- thus solving the second problem.

So, here's my proposal

1) Change the delta representation to index and store delta windows
separately

        DELTA ::= (("delta" FLAG ...) (OFFSET WINDOW) ...) ;
       WINDOW ::= DIFF SIZE CHECKSUM [REP-KEY REP-OFFSET] ;
       OFFSET ::= number ;
   REP-OFFSET ::= number;

The REP-KEY and REP-OFFSET in WINDOW are optional because, if the
differences between two file revisions is large enough, the diff could
in fact be larger than a compression-only vdelta of the text region. In
that case it makes more sense to compress the window than to store a diff.

2) Change the undeltifier to use the new structure

The undeltifier will stay essentially the same as it is now, except that
it will use OFFSET and REP-OFFSET to access the necessary bits directly.
The place where the delta combiner will fit stays the same, too.

The major addition comes after the text is reconstructed. Using some
suitable heuristic -- probably based on the number of jumps from the
representation to the fulltext, the size of the diff from the fulltext,
etc. -- we can decide to: a) replace the window with a single diff from
the fulltext, b) replace it with a compressed version of the region, or
c) do nothing.

The disadvantage of this proposal is, of course, more space used in the
repository. We can reduce the increase somewhat by compressing the
window index, and possibly improving the svndiff encoding. But I think
it's a fair price to pay, because this scheme reduces the number of disk
accesses, total memory use and average processing time needed to
reconstruct a region of text.

That's it. Let's see you find holes in my proposal. :-)

    Brane

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Received on Sat Oct 21 14:36:33 2006

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