> From: Phil Stracchino [mailto:ala...@metrocast.net] > Sent: Friday, August 13, 2010 7:51 AM > To: bacula-users@lists.sourceforge.net > Subject: Re: [Bacula-users] Dell PV-124T with Ultrium TD4, Hardware or > Software compression? > > On 08/13/10 04:10, 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. > > 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. > > Neither of these issues is applicable to LTO. The compression > algorithm > (which is a pretty good one) is defined in the LTO specification, and > the drive compresses data block-by-block, doing a trial compression of > each data block and writing whichever is the smaller of the compressed > and uncompressed version of that block to tape, flagging individual > blocks as compressed or uncompressed. > > -- > Phil Stracchino, CDK#2 DoD#299792458 ICBM: 43.5607, -71.355 > ala...@caerllewys.net ala...@metrocast.net p...@co.ordinate.org > Renaissance Man, Unix ronin, Perl hacker, Free Stater > It's not the years, it's the mileage. >
Remember also that if you are trying to minimize tape/disk/other backup media space used, and using encryption, you will need to use software compression. The FD compresses before encrypting; once encrypted, as noted above, the data is no longer compressible... ------------------------------------------------------------------------------ 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
