> From: Phil Stracchino [mailto:alaric AT metrocast DOT net]
> Sent: Friday, August 13, 2010 7:51 AM
> To: bacula-users AT lists.sourceforge DOT 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
>   alaric AT caerllewys DOT net   alaric AT metrocast DOT net   phil AT 
> co.ordinate DOT 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...

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