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?

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.  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).

If you are not getting something akin to this observed behaviour then your 
backup is not being limited by tape write speed, but by something else such as 
source input speed or compression speed.

Cheers,

Paul.


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