On Thu, Dec 26, 2002 at 09:26:39AM +1100, Craig Dewick wrote:
> 
> Any other little niggly gotcha's? I see that the DLT drives are capable of
> using a compression mode to increase data storage capacity, but if Amanda
> is already doing that in software using standard Unix or GNU tools, is
> there any point forcing compression on the drive as well? That would
> actually reduce the reliability of backups IMHO since the pre-compressed
> data being pushed out from Amanda would not be able to be compressed
> further by the drive itself.
> 
> I know this is a topic frequently discussed in regard to DDS drives, but
> this is my very first experience using a DLT drive, so the same comments
> may or may not apply...

It is not dds related.  It is a property of all (all meaning all I'm aware of)
loseless (hope that is the correct term for a "no lost data" scheme) compression
algorithms that they rely on detecting repeated patterns in the data being
compressed.  If those patterns are minimal, there is little compression.
And the overhead data added by the compressor may increase the size of the 
output.

Already compressed data is fairly random.  Thus it does not "double compress".

-- 
Jon H. LaBadie                  jon AT jgcomp DOT com
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