> Could someone techy from Dantz weigh in on this?

Wow -- where to begin?

1. Retrospect only copies unique files to the backup set, which does reduce
the amount of data copied to the backup set without reducing safety.
Typically, you can expect to only copy 70-80% of the data on a network,
depending on the numbers and types of computers and how they're used.

Our Snapshot technology then allows us to restore any computer on the
network exactly as it was -- even if some of the files did not originally
come from that computer.

2. Several years ago, when DDS-DC (DAT-1 with compression chip) drives first
came out, manufacturers had different claims for capacity. This caused great
customer confusion in the marketplace.

When DDS-2 drives arrived, Dantz sent a letter to all our tape contacts
informing them that, regardless of their marketing, we would only market a
2:1 compression ratio. Some agreed, others didn't. When customers contacted
us about the drives, we stuck with our belief that 2:1 was reasonable and
beyond that was a bit over the top. In addition, we worked with the media to
make sure articles reflected reality. For the most part, the strategy seems
to have worked. (No more "16GB!!" DDS-2 drives.)

3. Backup is all about time and space. If you had infinite time and infinite
space, you could always have everything backed up. Unfortunately, one has
neither infinite time nor infinite space. We have to make sure everything's
in balance. If we spent too many cycles getting slightly better compression,
our speed would suffer. While this may be OK for some folks, pushing the
balance too heavily towards either time or space could make a backup
application unmarketable.

(Note also that may tape technologies greatly suffer if data is not
continuously streaming to the drive. So, not only would time increase but
also it's likely that the amount of data the media could store would
actually *decrease* because of pad blocks caused by underruns of the media.
See http://www.dantz.com/index.php3?SCREEN=tn305, tech note #305 for more

4. Retrospect doesn't control the built-in hardware compression chip, other
than to turn off compression or turn it back up. We take advantage of the
technology if it's there. Otherwise, we use software compression which is
generally slower and not as efficient. If your computer is too slow compared
to your backup device, you may actually be trading off speed for marginal
gains in compression. Again, check out tech note #305.

5. Retrospect must rely on loss-less compression. We cannot get the same
compression as an MP3, jpg, etc., because it would be unacceptable to lose a
single bit of, for example, your payroll database. If there's a slight
artifact in an MP3 or jpg, it's hardly noticeable. If there's an artifact in
your payroll database, the results would be unpredictable, which defeats the
purpose of backing up in the first place.

I hope that about covers it!


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