[rdiff-backup-users] Understanding Session Statistics

[Dean Cording]

I'm in need of some help understanding the session statistics.  In particular, 
I need to know how much extra disk space each backup session takes.

According to the FAQ "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 understanding of rdiff-backup is that effectively nothing is ever deleted 
and therefore the backup size should always be increasing. For example, if I 
create a 1Mb file, back it up and then modify 500Kb worth of the file, then the 
next backup will grow by 500Kb consisting of the file in its current state and 
the 500Kb diff of original data that was modified. Even if I delete the file, 
the last version is still kept on the backup.

If that is the case, how do you explain the following session statistics where 
the TotalDestinationSizeChange is negative.

--------------[ Session statistics ]--------------
StartTime 1248023705.00 (Sun Jul 19 13:15:05 2009)
EndTime 1248024106.28 (Sun Jul 19 13:21:46 2009)
ElapsedTime 401.28 (6 minutes 41.28 seconds)
SourceFiles 45598
SourceFileSize 309310435 (295 MB)
MirrorFiles 45598
MirrorFileSize 311633827 (297 MB)
NewFiles 0
NewFileSize 0 (0 bytes)
DeletedFiles 0
DeletedFileSize 0 (0 bytes)
ChangedFiles 114
ChangedSourceSize 68692958 (65.5 MB)
ChangedMirrorSize 71016350 (67.7 MB)
IncrementFiles 115
IncrementFileSize 473538 (462 KB)
TotalDestinationSizeChange -1849854 (-1.76 MB)
Errors 0
--------------------------------------------------

Also, is there a way of roughly calculating how much data was transmitted 
between the client and server from the session statistics?

Thanks,

Dean