--On Thursday, April 21, 2005 12:27:20 -0400 Vicki Stanfield <vicki AT progeny 
DOT com> wrote:

> Jon LaBadie wrote:
> 
>> On Thu, Apr 21, 2005 at 10:49:00AM -0400, Vicki Stanfield wrote:
>>  
>> 
>>> If the dump is 28GB, should it not fit on a 20GB tape if it is 
>>> compressed. Aren't the tapes basically 20/40GB?
>>>    
>>> 
>> 
>> Not in my mind.  They are 20GB tapes.
>> I.e. they can hold 20GB of ones and zero bits.
>> 
>> What those ones and zeros represent "could be"
>> 40GB of data compressed ton 20GB of zeros and ones.
>> I.e. 50% compression.
>> 
>> But the level of compression varies greatly with
>> the type of data being compressed and the algorithm
>> being used.  In fact, the compressor used by your
>> tape drive can actually expand some types of data
>> rather than compressing it.
>> 
>> Using gzip as the software compressor I've tested data and
>> gotten 0, 3, 10, 25, 50, 75, even 90+ percent compression.
>> If the data in your dump compresses 50%, fine, it will fit
>> on a 20GB tape.  But if it only compresses 3%, I doubt it.
>> And if it was already compressed by gzip (or has lots of
>> gzipped files), it WILL EXPAND if the tapedrive compressor
>> works on it.
>> 
>>  
>> 
> Okay, I see that I asked my initial question badly. I know that you very 
> seldom if ever get 2-1 compression. I just meant that I thought that 1.4 to 1 
> wasn't too much to ask. Perhaps that is incorrect.

Depends on your data.   You may want to define both compressing and
non-compressing dumptypes and compress DLE's that can benefit from
it and not ones that don't (assuming you're using software compression).
> 
> Anyway, I am perfectly willing to stop using hardware compression altogether 
> but haven't figured out how to make the setting permanent yet using GNU-mt 
> 2.4.2.

On some drives you may have to set jumpers or dip switches on the drive
itself to disable it.  Even then, once a tape has been written in hw compression
mode it will sense that from a hidden header and keep using compression.
Gene usually jumps in here with his script to fix the hidden header, but I'm 
sure
you can find it in the list archives many times.

Frank

> 
> Vicki



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Frank Smith                                      fsmith AT hoovers DOT com
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