On Monday 02 June 2003 10:40, Gene Heskett wrote:
> If you then had to recover, you would need only the last 'dumpcycle'
> tapes.  Note that I didn't say 7 though, because you might be running
> a 5 day a week runspercycle, so that would be only the last 5 tapes.

Ok. Thanks - this just clicked for me.

> One would use the capacity the makers says it has when the compression
> is turned off.  Software is the prefered method, for 2 reasons.

My tapes say 40* - 80* GB (* = compressed). This is a DLT 8000 drive. This 
means that I have a 20GB capacity uncompressed? (Just to clarify.)  The 40 - 
80 GB means I can get between 2 to 4 times the storage with compression on?

> 1. gzip can normally outcompress the hardware compressors in most
> drives.  Night before last, the disklist and dumpcycle positioning
> was such that I put something over 10 gigs on a 4 gig DDS2 tape. Of
> course I don't do that every nite :)
>
> 2. amanda counts bytes sent to the tape.  If the hardware compression
> is on, then amanda has no clear view of the tapes capacity and will
> happily write till it hits EOT if you are feeding it already
> compressed files, like a whole dir full of rpm's or tar.bz2's, which
> will probably grow some in the compressor.  So you have to set the
> tapetype to something less than the makers propaganda claims & then
> (optionally, I've not been able to prove its effective) sacrifice a
> chicken.  If its turned off, then amanda has a pretty good view of
> what the tape can hold and will fill it reliably to 95+% without ever
> hitting the tapes EOT.
>
> There is an amtapetype utility in the distribution that can test that
> for you.  But start it early in the day, it takes a while to run.

I have already decided to use software compression (esp. for reason #2 above). 
I will have to compile the distro from the Amanda site as the Debian packages 
must not contain the amtapetype utility.

Thanks for everyone's help.

-- 
Brendon Colby
Systems Administrator
Midcontinent Communications