Craig White wrote:
> On Wed, 2009-01-07 at 17:57 +0100, Ralf Gross wrote:
>   
>> T. Horsnell schrieb:
>>     
>>> Ralf Gross wrote:
>>>       
>>>> Ferdinando Pasqualetti schrieb:
>>>>         
>>>>> I think you should use /dev/random, not /dev/zero unless hardware 
>>>>> compression is disabled in order to have more realistic figures.
>>>>>           
>>>> This wouldn't be a good idea, /dev/random or /dev/urandom are just too
>>>> slow in generating random data. To test the nativ speed of the  drive
>>>> creating a file from /dev/urandom and writing this file then from
>>>> tmpfs or a fast disk to the drive would be much better.
>>>>
>>>>
>>>> Ralf
>>>>         
>>> Personally, I'd use /dev/zero with compression turned off.
>>> Then there's *nothing* between the data-source and the tapedrive.
>>>       
>> Yes, but most people use hardware compresion with LTO drives. Sooner
>> or later he has to test the drive with compression.
>>     
> ----
> funny thing is that amanda developers are adamant that you disable
> hardware compression and use software compression instead.
>
> Obviously it takes longer and more cpu power to compress the files in
> software before storing them on the tape and if you leave hardware
> compression on and use software compression too, the files probably grow
> in size.
>
> Commercial backup software just seems to always use hardware
> compression.

"adamant" is probably an overstatement. The choice is up to the person 
configuring the backups. Turning off hardware compression allows more 
intelligent planning on Amanda's part. Using hardware compression 
introduces unknowns into tape usage and the size of data on the tape, 
since drives don't report back the results of their compression. There 
are always tradeoffs. If your cpu can't support the software compression 
and also maintain the data throughput, then it may make sense to use 
hardware compression and live with the planner being less accurate.

Most backup software leaves the responsibility for planning more in the 
sysadmin's hands. If the software is just writing data to the tape until 
it hits the end of tape and then asking for another, hardware 
compression is a logical choice.


-- 
---------------

Chris Hoogendyk

-
   O__  ---- Systems Administrator
  c/ /'_ --- Biology & Geology Departments
 (*) \(*) -- 140 Morrill Science Center
~~~~~~~~~~ - University of Massachusetts, Amherst 

<hoogendyk AT bio.umass DOT edu>

--------------- 

Erdös 4



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