Sun TAR

[Tim Ehrhart]

In article <342 at jc3b21.UUCP> larry at jc3b21.UUCP (Lawrence F. Strickland) writes:
>For tar tapes, it is common to use:
>
>	$ tar xvf /dev/rst0
>
>If you look in the /dev directory, you will also find a /dev/rst8 and a
>/dev/nrst8.  By using:
>
>	$ tar xvf /dev/rst8
>
>If someone could shed more light on this, I'd love to know...

Lawrence,
I'd love to take credit for some terrific insight, but alas I can only quote
from the SunOS st(4) man page:

/****************************************************************************/
DESCRIPTION
     The Sysgen tape controller is a SCSI  bus  interface  to  an
     Archive  streaming  tape drive.  It provides a standard tape
     interface to the device, see mtio(4), with some deficiencies
     listed  under  BUGS  below.   To  utilize the QIC 24 format,
     access the logical  device  that  is  eight  more  than  the
     default  physical  (QIC  11) device (that is, rst0 = QIC 11,
     rst8 = QIC 24).

Sun Release 3.4     Last change: 21 May 1986                    1

ST(4S)                    SPECIAL FILES                    ST(4S)

FILES
     /dev/rst[0-3]  QIC 11 Format
     /dev/rst[8-11] QIC 24 Format
     /dev/nrst[0-3] non-rewinding QIC 11 Format
     /dev/nrst[8-11]     non-rewinding QIC 24 Format
/****************************************************************************/


The REAL difference is that "st0" (QIC-11) uses 4 tracks, and "st8" (QIC-24)
uses 9 tracks. So the actual storage capacity is more than doubled. Here's how
I calculate tape capacity for these 1/4" puppies: (pseudo code)

	density = 1000 BytesPerInch (not changeable, as far as I know)
	ntracks = ("dev" == "st8" ? 9 : 4)
	tapelen = 300 ft., 450 ft., or 600 ft. (most popular sizes)
	fudge_factor = .9 (purely empirical)

	capacity = (((density * (tapelen * 12)) * ntracks) * fudge_factor)
	
Tim Ehrhart
Systems Programmer
SRI International
Menlo Park, CA