On Thu, 21 Apr 2005 at 10:49am, Vicki Stanfield wrote

> Well, I didn't remember to turn off software compression before this 
> run, so that is obviously a problem.
> 
> The whole tape definition is:
> # our tape definition
> define tapetype HP-C5683A {
>         comment "DDS-4 DAT tape"
>         length 16584 mbytes
>         filemark 452 kbytes
>         speed 2613 kbytes
> }
> 
> The length was set before I got here, but I googled around and it seemed 
> to be in keeping with the other HP-C5683A entries that I found. Is it 
> not correct?
> 
> If the dump is 28GB, should it not fit on a 20GB tape if it is 
> compressed. Aren't the tapes basically 20/40GB?

OK, so it looks like its time for the amanda and compression lecture.

Amanda can use software (i.e. the dumps are compressed with gzip as 
they're made) or hardware (your tape drive's internal) compression.  If 
you use software, you must be sure to turn off your tape drive's hardware 
compression (unless you're using LTO, which has a smart hardware 
compressor, apparently).

When using software compression, your tape length is set to the native 
capacity of your tapes.  The advantage is that amanda knows just how much 
the data compresses, and just how long your tapes are, and so can more 
efficiently plan how to put data on the tapes.  The disadvantage is the 
used CPU.

When using hardware compression, you must lie to amanda about how big your 
tapes are.  You must estimate how compressible your data is, and increase 
your tape length definition proportionally.  If you end up hitting EOT 
frequently, your data isn't as compressible as you thought, and you need 
to dial back the tape length.  Be aware that manufacturers' claims of 2X 
compression (or 2.6X in the case of Sony and AIT) are, in general, a pipe 
dream.  The advantage of hardware compression is the saved CPU cycles.

Clearer?

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