Artem Russakovskii's programming and technology blog

Updated: June 1st, 2008

Recently I ran into major problems using GNU diff. It would crash with "diff: memory exhausted" after only a few minutes trying to process the differences between a couple 4.5GB files. Even a beefy box with 9GB of RAM would run out of it in minutes.

There is a different solution, however, that is not dependent on file sizes. Enter rdiff – rsync's backbone. You can read about it here: http://en.wikipedia.org/wiki/Rsync (search for rdiff).

The upsides of rdiff are:

• with the same 4.5GB files, rdiff only ate about 66MB of RAM and scaled very well. It never crashed to date.
• it is also MUCH faster than diff.
• rdiff itself combines both diff and patch capabilities, so you can create deltas and apply them using the same program

The downsides of rdiff are:

• it's not part of standard Linux/UNIX distribution – you have to install the librsync package.
• delta files rdiff produces have a slightly different format than diff's.
• delta files are slightly larger (but not significantly enough to care).
• a slightly different approach is used when generating a delta with rdiff, which is both good and bad – 2 steps are required. The first one produces a special signature file. In the second step, a delta is created using another rdiff call (all shown below). While the 2-step process may seem annoying, it has the benefits of providing faster deltas than when using diff. In fact, you can pipe the first step into the second one without any trouble if you want, which is what I ended up doing).

Usage:

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$ rdiff signature ORIGINAL.txt SIGNATURE.sig
 
$ l -h SIGNATURE.sig
-rw-r--r-- 1 user users 25M 2008-04-23 22:32 SIGNATURE.sig
 
$ rdiff delta SIGNATURE.sig MODIFIED.txt DELTA.rdiff
 
$ l -h DELTA.rdiff
-rw-r--r-- 1 user users 82M 2008-04-23 22:36 DELTA.rdiff

And here's what you would do to reassemble MODIFIED.txt:

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$ rdiff patch ORIGINAL.txt DELTA.rdiff MODIFIED_REASSEMBLED.txt
 
$ l *.txt 
-rw-r--r-- 1 user users 4,471,493,588 2008-04-23 20:24 MODIFIED.txt
-rw-r--r-- 1 user users 4,471,493,588 2008-04-23 22:44 MODIFIED_REASSEMBLED.txt
-rw-r--r-- 1 user users 4,403,302,981 2008-04-23 20:20 ORIGINAL.txt

Just as expected – everything matches.

Now, all of this could have been done in one go like this:

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rdiff signature ORIGINAL.txt | rdiff delta -- - MODIFIED.txt DELTA.rdiff

As far as my usage of such a useful diff program, I was doing CSV dumps of certain fields from a MySQL database, like so:

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SELECT * FROM table WHERE some_condition='1' ORDER BY id DESC INTO OUTFILE '/home/dump/dump.csv' FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '"';

and then applying rdiff to get the [quite small] daily deltas.

That's all folks!