>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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