Question in occasion of SCSI devices (the tape store)

Alex Zbyslaw xfb52 at
Fri Jun 9 11:12:42 UTC 2006

Nick wrote:

>thanks for help Alex I found the best way for me to write info to the
>Can You answered on the last question How can I see free size on the
>tape or size what I filled?
My experience is that you can't really do either.  If anyone know 
better, I'd be glad to know!

If the tape is positioned at the end of data then you can issue an "mt 
-f /dev/nsa0 status" command but I have no idea how to interpret the 
result in terms of how much space is used or left.

Using dump, I write full backups to one tape which then gets put away.  
I just judge from how big the filesystems are how many will fit on one 
tape.  The if I do incrementals, they all go on one tape.  If the tape 
does run out, dump will just ask for a new one.

If you use a system like bacula, then it may keep track of how much data 
you have written, but with a compressing tape drive (which most are) you 
don't know exactly how that corresponds to how much data was written to 
tape because you can't accurately predict the compression ratio.  An 
estimate of 1.5:1 works OK for mixed filesystems, but if you having 
nothing but jpegs and mp3s (which are compressed already) then you may 
get 1:1.  For a database full of text you might get 2:1.

Not much help.  Sorry.


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