Ability to tell what has changed

[Garance AE Drosehn <gadcode@earthlink.net>]

I'm just getting started up with tarsnap, so this request might
be something that is already available but I haven't noticed how
to do it.

A few days ago I created an archive for a folder called cvsdepot.
Today I try a dry-run for creating another archive on that folder,
and tarsnap tells me:
                                       Total size  Compressed size
This archive                            20_468320         5_974899
New data                                   109878             7560

The thing is that I'm pretty sure I haven't changed anything in that
folder (although it is possible I did).  I do see that one subdirectory
named lily-rtools has been modified in the last two days. but none
of the files in all of cvsdepot have been modified.  And all (three)
of the files in lily-rtools add up to just 91272 bytes.  So I'm curious
what files tarsnap has determined to contain some new data.  In this
case this question is nothing more than idle curiosity, just because
I'm still in the process of learning tarsnap.  I'm confident there is
nothing wrong.   (fwiw, I also installed the new version 1.0.36 of
tarsnap since my earlier run)

But one of the reasons I like the idea of doing dry-runs is to see
if the amount of new data to backup seems "reasonable".  I've been
known to download or generate pretty huge files "temporarily", only
to forget about those files for years.  And there was one time that
a new version of the Tivoli Storage Manager caused my config file
to be handled differently, and I nearly backed up almost 1 TB of
data which I really did *not* want to be backed up to TSM.

Is there any way that tarsnap would tell me which files have new
data, at least when I'm doing a dry-run?  It would probably be
nice to have two options:  (1) a count of files which have new
data, (2) a list of the specific filenames.

And in a related question:  Is there way to do a dry-run and find
out that there is *no* new data to back up?  For some of the
folders that I back up, I do not want to create another backup if
nothing has changed.  Right now I check and assume that if the
new-data size is under 5000, then there's no point in creating
the backup.  Obviously a script could do this if tarsnap provides
a way to get the count of files with new data, but if it isn't
easy to get an exact count then it'd be nice to at least know
the difference between "something somewhere has changed" and
"nothing has changed".

-- 
Garance Alistair Drosehn            =  gadcode@earthlink.net
Senior Systems Programmer           or       gad@FreeBSD.org