On Thursday 08 August 2002 08:17, Martinez, Michael - CSREES/ISTM 
wrote:
>So what you're saying is that Amanda has no way of knowing how to
> calculate the amount of compression performed by the hardware,
> and therefore has no way to correctly estimate the amount of data
> to send to the tape.

Thats correct.  Please fix your email agent's lack of quoting as 
there is no way to ascertain in the message below, who said what.

>Michael Martinez
>System Administrator (Contractor)
>Information Systems and Technology Management
>CSREES - United States Department of Agriculture
>(202) 720-6223
>
>
>-----Original Message-----
From: Gene Heskett [mailto:gene_heskett AT iolinc DOT net]
>Sent: Thursday, August 08, 2002 8:14 AM
>To: Martinez, Michael - CSREES/ISTM; 'Scott Sanders';
>amanda-users AT amanda DOT org
>Subject: Re: To use or not to use DAT drive compression
>
>
>On Thursday 08 August 2002 07:44, Martinez, Michael - CSREES/ISTM
>
>wrote:
>>Why does hardware compression confuse Amanda?
>
>To state the not so obvious, amanda counts bytes *sent to the
> tape*. It does this after having applied any compression you may
> or may not have specified in your dumptype entry for that
> disklist entry.
>
>When the tape drives compressor is on, then amanda and the drive
> are speaking a different language as far as how much data can fit
> on a tape.  Since some data will expand when fed to that dumb RLL
> encoder in the drive (like a .bz2 file may double in size!)
> you'll be forced to use very conservative estimates for the tapes
> capacity in your tapetype specification, thereby wasting tape
> capacity.
>
>With drive compression turned off, and tapetype run against your
>drive and that data used as your tapetype, then amanda knows to
> the byte how nuch she can put on a tape.  By using a dumptype
> that varies from disklist entry to disklist entry, you can turn
> the software compression on and off to suit the data in the
> individual disklist entry.  Start out by useing 'compress
> server(client?) best' for everything and read the email you'll
> get when the run is done.  Any disklist entry that reports a
> compression of more than 100% is an entry containing data that
> even software compression can't compress so it got larger.  OTOH,
> I have 3 or 4 entries that regularly report a compression ratio
> of less than 20%, so they get really small on the tape.
>
>Hopefully this clarifies it a bit for you.

-- 
Cheers, Gene
AMD K6-III@500mhz 320M
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