[Borgbackup] Poor dedup with tar overlay

[John Goerzen]

Hi folks,

Long story, but I've been running borg over a 60GB filesystem for awhile
now.  This has been working fine.

I had a long thought regarding verifiability, and thought that I could
pipe an uncompressed tar of the same data into borg.  This should,
theoretically, use very little space, since tar has some metadata
(highly compressible), and NULL-padded blocks of data.  These data
blocks would be exact matches for what's already in the borg repo.

To my surprise, however, this experiment consumed 12GB after compression
and dedup.  Any ideas why that might be?

My chunker params are at the default.

Thanks,

John