[rdiff-backup-users] interpreting FAQ.html#statistics

[Jeffrey 'jf' Lim]

as per http://rdiff-backup.nongnu.org/FAQ.html#statistics:

=======
TotalDestinationSizeChange is the number of bytes the destination
directory as a whole (mirror portion and rdiff-backup-data directory)
has grown during the given rdiff-backup session. This is usually close
to IncrementFileSize + NewFileSize - DeletedFileSize +
ChangedSourceSize - ChangedMirrorSize, but it also includes the space
taken up by the hardlink_data file to record hard links.
=======

My question is about the (estimation) formula for
TotalDestinationSizeChange. Specifically, the "- DeletedFileSize". I
dont propose to say that this is exactly how rdiff works, but I've
worked with the standard 'diff' before - and if something gets
deleted, the diff file doesnt exactly shrink. It, in fact grows,
because it has to record the data that has now been taken out.

So my question is this: why take away the DeletedFileSize? I can
definitely understand "+ IncrementFileSize", and "+ NewFileSize",
but... as mentioned, shouldn't a diff grow in size as well when you
take stuff out?

thanks,
-jf

--
"Every nonfree program has a lord, a master --
and if you use the program, he is your master."
    --Richard Stallman

"It's so hard to write a graphics driver that open-sourcing it would not help."
    -- Andrew Fear, Software Product Manager, NVIDIA Corporation
http://kerneltrap.org/node/7228