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 [ala...@metrocast.net]
Sent: Friday, August 13, 2010 9:50 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.
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