You:
> On Aug 13, 2010, at 2:23 PM, Dietz Pröpper wrote:
> > You:
> >> On Aug 13, 2010, at 4:10 AM, Dietz Pröpper wrote:
> >>> IMHO there are two problems with hardware compression:
> >>> 1. Data mix: The compression algorithms tend to work quite well on
> >>> compressable stuff, but can't cope very well with precompressed
> >>> stuff, i.e. encrypted data or media files. On an old DLT drive (but
> >>> modern hardware should perform in a similar fashion), I get around
> >>> 7MB/s with "normal" data and around 3MB/s with precrompessed stuff.
> >>> The raw tape write rate is somewhere around 4MB/s. And even worse -
> >>> due to the fact that the compression blurs precompressed data, it
> >>> also takes noticeable more tape space.
> >> 
> >> Those problems affect software compression, too.
> > 
> > Hmm, I can't reproduce them with gzip on the data in question.
> 
> Maybe, then, your job is not I/O bound?

Sorry, I was a little unclear - with gzip, file size does not get bigger, 
regardless of the entropy of the data compressed.

> If backup is limited by the speed at which you can write to tape then it
> logically follows that you will get the observed behaviour you mention
> above.  More compressible source data will lead to faster backup rates
> because those data will compress to less bits that need to be written
> to tape.

Of course.

> Conversely, if the compression algorithm does not take steps
> to guard against growth due to compressed input, backup speeds will
> fall below nominal write speeds with such data because the source data
> will result in more bits to be written, which will take longer
> (relative to the source).

That's what I observe with my ancient DLT drive.

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