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. ------------------------------------------------------------------------------ This SF.net email is sponsored by Make an app they can't live without Enter the BlackBerry Developer Challenge http://p.sf.net/sfu/RIM-dev2dev _______________________________________________ Bacula-users mailing list Bacula-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/bacula-users