On Thu, 20 Jan 2005 at 9:09am, Neil Marjoram wrote

> I have just completed a tapetype on my Storedge L9 with DLT 8000 again, 
> and I'm a little confused.
> 
> The DLT 8000 drive can take 80GB compressed, the results return 40GB.

Stop right there.  That 80GB number is just marketing, and generally 
*very* optimistic.  How much you can actually get on the tape using 
hardware compression varies drastically with your data.  If you're taping 
GBs of text files, sure, it'll compress down a lot.  If you're taping GBs 
of .tar.gz archives, it'll *expand* going through the hardware compresser.
 
> define tapetype DLT-L9 {
> comment "DLT 8000 - Storedge L9"
> length 40000 mbytes
> filemark 22 kbytes
> speed 1770 kbytes

If you're going to use hardware compression, you have to lie to amanda 
about your tapelength -- amanda will never schedule more than tapelength 
worth of data on a tape.  So, guess how compressible your data is, and 
increase your tapelength by that amount.  If you start hitting EOT too 
often, dial back your tapelength.

As an aside, this is why a lot of folks prefer using software compression.  
Then you don't have to lie to amanda, amanda will keep track on a per DLE 
basis exactly how compressible your data is, and you can turn off 
compression on DLEs where it doesn't buy you much.  Of course, some folks 
can't afford the cycles, and in that situation hardware compression is 
just fine.

-- 
Joshua Baker-LePain
Department of Biomedical Engineering
Duke University