On Fri, Jun 02, 2006 at 08:14:28AM -0700, Joe Donner (sent by Nabble.com) 
enlightened us:
> Firstly - thank you for the replies.
> 
> Secondly - I'm now more confused than before!
> 
> Is my understanding correct then that I can conceivably use this tapetype
> definition, but that I probably shouldn't expect to be backing up more than
> approximately 160GB to a tape?  Or, more to the point, approximately 135GB
> in this case?  And that I should really disable compression (but that this
> isn't necessarily required) and accept the tape's capacity of around 160GB?
> 
> I'm new to Linux (Redhat ES 3 in our case) and Amanda, so wouldn't mind
> things being spelled out to me - if you wouldn't mind doing that.

Not at all.

1. Tapes for your tape drive hold 160GB of data. Period.

2. When you try to compress data that is already compressed, it expands.
   This is evidenced by the fact that amanda was only able to write 135GB of
   compressed data to your tape drive: when it got through the tape
   compressor, it expanded to 160GB.

3. Amanda works best when using software compression, because it knows how
   many bytes were sent to the drive are how many will be written to tape.
   If you wish to use hardware compression, you have to "lie" to amanda to
   tell it what tape size you think you'll have. SDLT's Marketing Department
   would have you believe that is 320GB. The real answer is it depends on
   what you're backing up. If it's directories of .jpg images and tarballs,
   you won't get anywhere near 320GB. If it's a bunch of text files, you
   might even exceed 320GB. The fact of the matter is it's a guessing game
   with hardware compression. Using software compression, amanda will
   compress your data before sending it to the tape drive, so it knows how
   to fit 160GB on the tape. That 160GB, when expanded, could be anywhere
   from 160GB - 300GB or more. Again, depends on your data.

4. You can use the tapetype as is, and amanda will base logic decisions on
   that information, but once in backup mode, amanda writes to tape until it
   hits an error (generally End Of Tape (EOT), but sometimes there are media
   errors, etc.). 
   
Does that help?

Matt

-- 
Matt Hyclak
Department of Mathematics 
Department of Social Work
Ohio University
(740) 593-1263