On Sun, Jan 25, 2004 at 04:35:17PM +1100, Jobst Schmalenbach wrote:
> 
> Folks,
> I am not clear about compression within amanda.
> 
> I bought a new tape drive (Certance CD72) and read the instructions
> for amtapetype. Now amtapetype suggests to turn off hardware
> compression, *THAT* I do understand (growing binary).
> 
> Now I want to use *hardware* compression.
> The advantages are:
> 
>  * faster
>  * crc errors only fail the current block only not
>    the entire archive.
> 
> 
> If I turn hardware compression ON how does AMANDA know
> (well at least the planner) how much tape is available
> if the amount is specified for NOT COMPRESSED?
> 

You guess and enter that in the length field.
Pick a number between what you measured and what the
manufacturer claims.

Amanda tracks software compressibility for the last
three runs (under curinfo) on a DLE by DLE basis.
If you have those results, hardware compressibility
should be somewhat similar.

I say guess because I just looked at my recent numbers,
some DLE's only compressed a couple of percent.  Others
compressed to just 1/5th of their original size.  What
will be the compressibility of the data on any particular
nights run will vary greatly with which DLE's are dumped
at level 0.

jl

BTW your reasons are all valid for considering HW compression.
I think more amanda sites use HW compression than most of us
realize.  Just be aware that there is no way for amanda, or
any other software, to know how much data will fit on a tape
using HW compresssion.  Nor, to my knowledge, to know how much
tape is used/available after a tape file has been written.

-- 
Jon H. LaBadie                  jon AT jgcomp DOT com
 JG Computing
 4455 Province Line Road        (609) 252-0159
 Princeton, NJ  08540-4322      (609) 683-7220 (fax)