One (compound) word: Lossless
The compression methods that you are lusting after introduce errors
(artifacts) into the resulting decompressed data. This may be acceptable
for sound, photos and video but to totally unacceptable for storage of
system files, programs and most other data.
High levels of compression can only be achieved if the compression method
has embedded knowledge of the semantic content of the data. A general purpose,
lossless compression engine will not be able to compress random and
packed data (no white space or zeroed buffer data). On the other hand, even
a simple compression method will achieve high compression ratios if the data
is highly repetitive or void of information. This is why "your mileage varies."
Don't be too hard on the tape drive developers. They are working against some pretty
solid limitations in the mathematics of what's possible. On the other hand,
they are working hard to speed up the methods and the chips in which they are
embedded in order to keep up with increasing interface and transport speeds.
Future gains in capacity will come from denser on-media data and longer/wider reels
but not, it is thought, from innovations in compression methods.
Of the next startup IPO touting a revolutionary new compression algorithm,
be sceptical...very sceptical.
Doug.
>One word: Compression.
>
>Native capacities are with no compression. For example, your 12GB drive can hold up
>to 12GB of un-compressed data. Normal hardware compression can get up to 2:1
>compression, giving a total maximum of 24GB.
>
>Reality check, I've been getting about 27GB onto a 20GB/40GB drive. This is typical.
>You should be able to get 17-18GB on the drive on average.
>
>Warning, my rant follows:
>
>What I don't get is that they have not improved the compression on tape drives all
>that much. I mean, compression programs regularly get 4:1 to 24:1 compression
>depending on the data. Heck, a JPEG image is typically 12:1 compression.
>
>Yes, I do know that a compressed file usually cannot be compressed any further.
>Throw a JPEG file into a ZIP or Stuffit program and you might gain 1% at best.
>However, the majority of files on a computer are not compressed.
>
>I once took a System Folder that was 220MB in size and compressed it using Stuffit
>down to 28MB. That's almost 8:1 compression. Yes, the process to some time to do,
>but maybe it should be an option. I'd like to get another 10GB out of this tape
>drive as there are weeks when the tape runs out on Thursday night needing just
>another 1-2 MB to finish the week. I stopped backing up my laptop on the regular
>backups just to make room. I really hate added more tapes to a set. I only backup 11
>machines on this. If I had a choice of 4:1 compression, and could expect a yield of
>little better than 2:1, everything would fit and I could add another machine to the
>backup.
>
>>Hello,
>>
>>I'm new to this list and subscribed because I have a question I hope one of
>>you will be able to answer for me.
>>
>>I'm using Retrospect 4.2 for Mac, backing up a total of three Mac Servers to
>>an APS DAT drive. The tapes I use are Sony DDS3 125P which state a Native
>>(I'm not sure what Native means) capacity of 12.0 GB.
>>
>>After looking at the log of how much data is being backed up, it added up to
>>14.3 GB. How can this be? It seems that the tape shouldn't be able to handle
>>it but I receive no errors.
>>
>>Any explanations? Suggestions?
>>
>>thank you,
>>
>>-matt
>>
>>
>>
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>
>
>
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