Large capacity tape drives (VHS) and dump

[Marc Greisen]

Reply-to: marcg at phred.UUCP (Marc Greisen)



Software involved: Ultrix 2.0

A number of months ago we were able to purchase one of the large
capacity tape drives that have recently come on the market.  The one we
bought was Digi-Data's Gigastore which uses VHS format tapes.  The other
one is Exebyte's 8mm unit.

We have been merrily using this unit just fine, but have recently
recognized a problem.  It appears that dump is unable to handle the
idea of a tape that can store 2.5 gigabytes of data.

We first came to this conclusion when dump would calculate that it
needed almost a whole tape for a 360 megabyte partition.

We have been giving dump these flags.
	d 2,000,000  - density expressed in bits per inch
	s 807        - length of tape in feet

these numbers were obtained by using capacity numbers supplied by Digi-Data.
A tape is 807 feet long and has a capacity of 2.5 gigabytes.  Therefore

             2.5 gigabytes
      -------------------------  =  2,165,584 bits per inch
	807(feet) * 12(inches)


Since we are a binary only site, we assume that dump uses ints for its 
variables. So given that
MAXINT is:

           2^32 - 1
          ----------  = 2,147,483,647
               2

we can see that if dump does all of it's the multiplication before 
any division, and we want to stay within MAXINT, we get: 

 2,000,000 bpi * 12 inches * 89 feet    2,136,000,000
 -----------------------------------  = -------------  = about 254 meg
                                         8 bits/byte   


Now as you can see I haven't been using 89 feet in my dump command, but
807 feet.  Therefore it seems likly we are getting an overflow condition 
that is not being trapped and wouldn't you know, it all seemed to work.

At some point dump seems to do a calculation and come up with 10240
byte blocks.  Irregardless there still appears to be a problem centered
around 32 bit ints.

So my question:  

     Has anyone else, given the popularity of these kind of
     tape devices, seen/fixed/worked around this problem?

     Anyone know how dump actually does it's calculations so
     I can maximize the dump data?


Marc Greisen
Physio-Control Corp.,
...tikal!phred!marcg
(206)867-4535