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.

LTO takes steps to ameliorate the effects of pre-compressed/high entropy input 
data by allowing an output block to be prefixed as being uncompressed.  So, if 
input data would cause a block to grow due to compression, it can output the 
original input itself, with the block prefixed, meaning only a very tiny 
percentage increase in tape usage for stretches of high-entropy input.  
Software compression also takes steps to limit growth in output due to 
highly-compressed input.

> 2. Vendors: I've seen it more than once that tape vendors managed to break 
> their own compression, which means that a replacement tape drive two years 
> younger than it's predecessor can no longer read the compressed tape. 
> Compatibility between vendors, the same.
> So, if the compression algorithm is not defined in the tape drive's 
> standard then it's no good idea to even think about using the tape's 
> hardware compression.

I agree with point 2, however I believe the trend has been to move towards 
using algorithms defined and documented in published standards for the very 
reasons you state.

Cheers,

Paul.


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