>Matt, if you think you're actually getting 4:1 real world compression out of
>a modem, I suggest you read some of the research on the subject.

I never said that... What I did say:

>Yes, I know that advertised and what you really get are totally
>different, but all I know is that if something is advertised at 4:1,
>it will be more likely to get at least 2:1 than 2:1 is likely to.

If I attempt 4:1 compression and only get 2-3% on some files, but on 
some files I got 2-3:1, what did I lose or gain?

If I attempt 2:1 compression and only get 2-3% on some files, but on 
some other files I got 1.2 - 1.5:1, what is the advantage?

Keep reading before replying...

4:1 compression under V.42bis is an on the fly compression.  Granted 
the data rates under that specification are a lot less.  (up to 1.5Mb 
vs up to 320MB).  (V.42bis was also written for use on T1.)

What I don't understand is why there is not an option for greater 
compression even if the cost is speed.

For example, let's say I have 4 computers holding a total of 8GB to 
backup over the weekend and my tape drive is only 4GB without 
compression.  With 2:1 compression I am  guaranteed it will not fit. 
With 4:1 compression it might.  It all depends on the data.  I really 
don't care if it takes 24 hours to do it, because the backup goes 
over the weekend.  It should be an option.

BTW - if you backup JPEGs, GIFs, MPEGs, and several others you will 
not compress these files at all. In fact, any compression done on 
them usually results in a larger file.  These are compressed already 
with a compression scheme that is far better than anything else for 
the specialized purpose they do.  Yes, I know about the data losses 
with certain compression schemes, like JPEG will discard information. 
That is totally unacceptable with backup systems.

If you are backing up a normal desktop system, you should be getting about
1.5:1 compression on average.  If you have systems with a lot of 
graphic files or already compressed files, then yes, the compression 
will be closer to 1:1 (non-compression).

The point is, that you try to achieve 200% and only get 150%, where 
as if you try for 400% and get 200%, you've done better.  If the 
sacrifice was speed, then have a checkbox list that says:

   O  Software Compression 4:1
   O  Software Compression 2:1
   O  Hardware Compression

And let the users decide.

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