Re: [Bacula-users] Dell PV-124T with Ultrium TD4, Hardware or Software compression?
2010-08-13 10:09:16
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 AT lists.sourceforge DOT net
https://lists.sourceforge.net/lists/listinfo/bacula-users
|
<Prev in Thread] |
Current Thread |
[Next in Thread>
|
- [Bacula-users] Dell PV-124T with Ultrium TD4, Hardware or Software compression?, Mike Hanby
- Re: [Bacula-users] Dell PV-124T with Ultrium TD4, Hardware or Software compression?, Phil Stracchino
- Re: [Bacula-users] Dell PV-124T with Ultrium TD4, Hardware or Software compression?, Mike Hanby
- Re: [Bacula-users] Dell PV-124T with Ultrium TD4, Hardware or Software compression?, Dietz Pröpper
- Re: [Bacula-users] Dell PV-124T with Ultrium TD4, Hardware or Software compression?, Peter Zenge
- Re: [Bacula-users] Dell PV-124T with Ultrium TD4, Hardware or Software compression?, John Drescher
- Re: [Bacula-users] Dell PV-124T with Ultrium TD4, Hardware or Software compression?, Dietz Pröpper
- Re: [Bacula-users] Dell PV-124T with Ultrium TD4, Hardware or Software compression?, Phil Stracchino
|
|
|