Thanks all, this is good information.

I'm in the process of finishing a full 6TB backup, will see how hardware 
compression performed.
________________________________________
From: Phil Stracchino [alaric AT metrocast DOT net]
Sent: Friday, August 13, 2010 9:50 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.

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