Re: [rdiff-backup-users] Understanding Session Statistics

[Patrick Nagel]

Hi,

On 2009-07-20 08:07, Dean Cording wrote:
> 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
> --------------------------------------------------

Just a guess: maybe because the diffs are gzip compressed? If you make a
change, and the reverse diff to the last state of the changed file is
compressible, it could result in the reduction of the total size.

Patrick.

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