Re: Tarsnap GUI shows 0 data archived or backed up

[Graham Percival <gperciva@tarsnap.com>]

On Thu, Sep 07, 2017 at 08:21:42AM -0000, amar@sdf.org wrote:
> I believe too much feedback in my last mail might have been off-putting
> and judgemental (and kinda unsolicited advice) :-) Or not.

Hi Amar,

No, your previous mail wasn't too judgemental at all!  Sorry for the delay.

> > I am backing up 1.45 GB and I see the usage are 1.07 GB. Yes, there are
> > other archives but they are in KBs. Even after the first archive I believe
> > it was either 1.07 GB or something very close to it - it was > 1 GB.
> >
> > So, can I make it smaller than 1.07 GB? Am I doing something wrong? I can
> > explain what kind data are part of those 1.45 GB (can give a breakup too
> > if needed).
> >
> > Can I compress it more, make de-duplication more aggressive? I have used
> > the default config that came with the app.

The short answer is no, you cannot alter any parameters of Tarsnap's
deduplication and compression.

At first glance, going from 1.45 GB to 1.07 GB looks reasonable.  It very much
depends on what data you're backing up, of course!  If you'd like to give me
more details about what you're backing up (either publicly or privately), I
could try to make a better guess.

Tarsnap's deduplication is primarily useful when you archive the same
directories later on -- it will only upload data that's changed (plus a little
bit of metadata).  For example, one of the tarsnap.com servers generates an
archive every hour; the sum of all those archives is 96000 GB, but thanks to
deduplication that's reduced to 56 GB, and after compression (that particular
data) is 16 GB:
http://www.tarsnap.com/deduplication.html

Note that deduplication works best if your data is not already compressed:
http://www.tarsnap.com/tips.html#compression

I'm guessing that your data includes some photos?  Most image formats are
already compressed, so Tarsnap's deduplication and compression won't do much to
their file size.


If you'd like a technical discussion about the details of deduplication, see
https://www.tarsnap.com/download/EuroBSDCon13.pdf
One detail that isn't covered there is that after deduplication, each block is
compressed with zlib.  There's technical info about zlib and the DEFLATE method here:
https://zlib.net/zlib_tech.html

Cheers,
- Graham