Weird behaviour of AIT-3 and (g)tar

[Tulio Guimarães da Silva]

Hi Gary,
   ouch! That´s quite disappointing... :( We had already noticed this 
kind of behaviour with DDS-* tapes, but we got some progress varying the 
block size... and yup, I´m really using gzipped data. :S
  For AIT-3, however, i thought this hardware compression was something 
about using lower tape´s phisical-rolling speeds or alikes, but I could 
never really find anything concrete about the methods... the only one 
thing I found was they could use "variable block sizes", but that´s all. 
Again, not many details. Anyway, I´m giving up the idea of compression 
for now.
  If something, I´m noticeing that (at least with -b 10) it becomes (a 
lot) slower with time, but I guess this would be more of a question to 
the -performance list.
  Add: while writing this message, I remembered to check the 700V´s 
"Product Specification Manual", and they mention something about 
dual-partitions, but it seems something that needs to be implemented at 
driver level, since it includes SCSI commands. In this case, I would 
need to format the tape as a 2-partition one... any clue about if and/or 
how that works on FreeBSD?
  Thanks again,

Tulio

Gary Corcoran wrote:

> Tulio Guimarães da Silva wrote:
>
>> Hello again,
>>
>>    I´m having some trouble putting a Sony SDX-700V SCSI AIT-3 unit to 
>> work on FreeBSD 5.3-RELEASE.
>
> ...
>
>>  Besides the speed, hardware compression seems to not being funcional 
>> either. I already tried every 4 possible dip switch setting for 
>> compression, but I am still not able to transfer a 180GB archive to a 
>> (should-be) 260GB medium.
>
>
> I can't help you with most of your problems, but regarding the 
> "compression"...
>
> I would guess that if you have 180GB to backup, it's not all text.  :)
> When I last used a tape drive years ago, when writing to a 2GB tape
> that would supposedly hold 4GB compressed, I could fit only about 1.9GB
> before the tape was full.  Turning off hardware compression, I could fit
> 2GB.  The problem was that I was saving already compressed multimedia 
> files,
> and the tape drive's "compression" just added overhead and took up 
> more space.
> So unless you're backing up text or similar files, don't believe the
> marketing hype about getting 2x the amount onto your tapes...
>
> Gary