Alan Brown wrote:
> On Wed, 7 Jan 2009, Chris Hoogendyk wrote:
>   
>> 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.
>>     
>
> Part of the preference for no hardware compression in Amanda and other
> packages is that a lot of them assume fixed tape sizes, so the only
> thing that hardware compression gets you is less time to write to that
> maximum.
>
> ie: Program assumes tape is 33Gb. When 33Gb is reached the program
> demands the tape be changed, even if the tape is only physically half
> used thanks to hardware compression
>
> Most of these types of programs have an all-or-nothing approach to
> restores. This is not exactly the bacula model.
>
> Even with hardware compression switched off, spooling is likely required
> to prevent shoeshining of modern tape drives. Spooling hardware MUST be
> fast enough to keep up. Switching on hardware compression increases those
> requirements but the budgetary increase is minor.


Most of your batch of replies were on the mark. This one is at least in 
part incorrect and confusing.

 From the linux man page for amanda.conf:

"Once the backups start, Amanda will continue to write to a tape until 
it gets an error, regardless of what value is entered for [tape] length"

In other words the length entered in the tapetype definition is only 
used for planning and scheduling. The "error" will normally be the end 
of tape "error", which allows for a variable amount of data actually 
being written to the tape, which would include allowing for variable 
hardware compression results.

Also, I have no idea what you mean by "all or nothing restores". That's 
just wrong.


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

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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