John Drescher wrote:
>> DAT/DDS are inherently unreliable, apparently due to the narrow tape
>> width. (8mm helical scan tape formats such as AIT are fine)
>>
>>     
> Since AIT was mentioned. I would like to caution people about this.
> The drive makers assume a 2.61:1 compression instead of a 2:1 like
> every one else. Sure the compression may be a little better but I
> doubt it is that much and this better compression will not help you
> with uncompressable files. If you are not hitting 2:1 with the other
> drive technology you will not hit 2.61:1 with AIT.

Manufacturer's claims for compression should always be taken with a 
grain of salt (also known as knowledge) for all drive types.

I can contrive a file that will give me unbelievably huge compression 
ratios. But it's not realistic. Real data can be all over the map. You 
just have to know the native raw capacity of the tapes, and then go by 
experience with your own data. Are you dealing with a microscopy lab 
that is saving lots of digital images in a format that is already 
compressed? Or are you dealing with a business office with lots of word 
or excel documents that are text based with lots of redundancy? You may 
do worse than native capacity if you try to compress compressed data in 
the first case (this is somewhat algorithm dependent). You may do better 
than advertised compression ratios if all of your stuff is word documents.

Typically, I get only 14G or 15G on the 12/24G DDS/3 tapes. I have yet 
to fill an AIT5 tape, so I don't yet know what it will be when I hit 
that limit. I can run all the tests I want, but what really matters is 
how my actual data has evolved when I hit that limit.


---------------

Chris Hoogendyk

-
   O__  ---- Systems Administrator
  c/ /'_ --- Biology & Geology Departments
 (*) \(*) -- 140 Morrill Science Center
~~~~~~~~~~ - University of Massachusetts, Amherst 

<hoogendyk AT bio.umass DOT edu>

--------------- 

Erdös 4



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