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-*- Text -*-
Xdelta -- A binary delta generator
Announcing version 1.1.2 of Xdelta. Xdelta is an application program
designed to compute changes between files. These changes (deltas) are
similar to the output of the "diff" program in that they may be used
to store and transmit only the changes between files. However, unlike
diff, the output of Xdelta is not expressed in a human-readable
format--Xdelta can also also apply these deltas to a copy of the
original file. Xdelta uses a fast, linear algorithm and performs well
on both binary and text files.
Xdelta 1.1.2 is a stable, maintenence release. New, ongoing work on
Xdelta has focused on a new storage system with features similar to
the RCS command set. For more information on new development, see the
Xdelta-2.0 release series at http://xdelta.sourceforge.net.
Xdelta was designed and implemented by Joshua MacDonald. The delta
algorithm is based on the Rsync algorithm, though implementation and
interface considerations leave the two programs quite distinct. The
Rsync algorithm is due to Andrew Tridgell and Paul Mackerras.
To compile and install Xdelta, read the instructions in the INSTALL
file. Once you have done this, you should at least read the first few
sections of the documentation. It is available in info format. All
documentation is located in the doc/ subdirectory.
This release, version 1.1.2, and future releases of Xdelta can be
found at http://xdelta.sourceforge.net.
Xdelta is released under the GNU Library Public License (GPL), see the
file COPYING for details.
There is mailing list for announcements:
xdelta-announce@lists.sourceforge.net
you can subscribe to the mailing list or file bug reports through
Sourceforge at:
http://sourceforge.net/projects/xdelta/
Comments about Xdelta can be addressed to the following addresses:
jmacd@cs.berkeley.edu
The man page describes how to use Xdelta in more detail:
NAME
xdelta - Invoke Xdelta
SYNOPSIS
xdelta subcommand [ option... ] [ operand... ]
DESCRIPTION
Xdelta provides the ability to generate deltas between a pair
of files and later apply those deltas. It operates similar to
the diff and patch commands, but works on binary files and does
not produce a human readable output.
Xdelta has three subcommands, delta, patch, and info. Delta
accepts two file versions and produces a delta, while patch
accepts the original file version and delta and produces the
second version. The info command prints useful information
about a delta. Each subcommand will be detailed seperately.
Gzip processing
Attempting to compute a delta between compressed input files
usually results in poor compression. This is because small
differences between the original contents causes changes in the
compression of whole blocks of data. To simplify things,
Xdelta implements a special case for gzip(1) compressed files.
If any version input to the delta command is recognized as
having gzip compression, it will be automatically decompressed
into a temporary location prior to comparison. This temporary
location is either the value of the TMPDIR environment
variable, if set, otherwise "/tmp".
The Xdelta patch header contains a flag indicating that the
reconstructed version should be recompressed after applying
the patch. In general, this allows Xdelta to operate
transparently on gzip compressed inputs.
There is one potential problem when automatically processing
gzip compressed files, which is that the recompressed content
does not always match byte-for-byte with the original
compressed content. The uncompressed content still matches,
but if there is an external integrity check such as
cryptographic signature verification, it may fail. To prevent
this from happening, the --pristine option disables automatic
gzip processing.
MD5 integrity check
By default, Xdelta always verifies the MD5 checksum of the
files it reconstructs. This prevents you from supplying an
incorrect input during patch, which would result in corrupt
output. Because of this feature, you can feel confident that
patch has produced valid results. The --noverify option
disables MD5 verification, but this is only recommended for
performance testing.
Compressed patch format
Xdelta uses a fairly simple encoding for its delta, then
applies zlib compression to the result. You should not have to
post-compress an Xdelta delta.
Delta
The delta subcommand has the following synopsis:
xdelta delta [ option... ] fromfile tofile patchout
Computes a delta from fromfile to tofile and writes it to patchout
Patch
The patch subcommand has the following synopsis:
xdelta patch [ option... ] patchin [ fromfile [ tofile ]]
Applies patchin to fromfile and produces a reconstructed
version of tofile.
If fromfile was omitted, Xdelta attempts to use the original
fromfile name, which is stored in the delta. The from file
must be identical to the one used to create the delta. If its
length or MD5 checksum differs, patch will abort with an error
message.
If tofile was omitted, Xdelta attempts to use the original
tofile name, which is also stored in the delta. If the
original tofile name already exists, a unique filename
extension will be added to avoid destroying any existing data.
Info
The info subcommand has the following synopsis:
xdelta info patchinfo
Prints information about patchinfo and the version it
reconstructs, including file names, lengths, and MD5 checksums.
Options
-0..9 Set the zlib compression level. Zero indicates no
compression. Nine indicates maximum compression.
-h, --help
Print a short help message and exit.
-q, --quiet
Quiet. Surpresses several warning messages.
-v, --version
Print the Xdelta version number and exit.
-V, --verbose
Verbose. Prints a bit of extra information.
-n, --noverify
No verify. Turns off MD5 checksum verification of the
input and output files.
-m=SIZE, --maxmem=SIZE
Set an upper bound on the size of an in-memory page
cache. For example, --maxmem=32M will use a 32 megabyte
page cache.
-s=BLOCK_SIZE
Set the block size, unless it was hard coded (20% speed
improvement). Should be a power of 2.
-p, --pristine
Disable the automatic decompression of gzipped
inputs, to prevent unexpected differences in the
re-compressed content.
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