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Top Forums UNIX for Dummies Questions & Answers passthrough devices vs. named devices Post 302202331 by thumper on Wednesday 4th of June 2008 02:41:35 PM
Old 06-04-2008
passthrough devices vs. named devices

I am having trouble understanding the difference between a passthrough device and a named device and when you would use one or the other to access equipment.

As an example, we have a tape library and giving the command
"camcontrol devlist" gives the following output:

akx[22]# camcontrol devlist
<SPECTRA 215 1014> at scbus3 target 3 lun 0 (pass4,ch0)
<SONY SDX-300C 04c7> at scbus3 target 8 lun 0 (pass5,sa0)

The SPECTRA 215 is the tape library the SONY SDX is the tape drive in the library.
To address the tape library a command such as "mtx -f /dev/pass4 command" is used, however to address the tape unit then /dev/sa0 would be used as in
"/sbin/dump -0uaf /dev/sa0 /dev/da4s1e".

Could someone give me a clue on when you address a passthrough device and when you address a named device, or at point me to some documentation that might clear this up?

Thanks
Thumper
 

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mt(1)							      General Commands Manual							     mt(1)

Name
       mt - magnetic tape manipulating program

Syntax
       mt [-f tapename] command [count]

Description
       The command permits the operation of a magnetic tape drive.

Options
       The  -f	flag  option  uses  the specified tape device (next argument) in place of either that tape device defined by your TAPE environment
       variable (.login or .profile) or /dev/nrmt0h.

       Some operations may be performed multiple times by specifying count.  By default, performs the requested operation once.

       The command argument defines the operation to be performed.  Only as many characters as are required to uniquely identify a command need be
       specified.

       The following is a list of commands:

       bsf		   Backspace count files.

       bsr		   Backspace count records.

       cache		   Allows to use the cache buffer on a tape drive that has the cache buffer feature.

       clhrdsf		   Clear  hardware/software problem.  Works with tape drives which use the TMSCP tape controller interface This command is
			   restricted to root access only.

       clserex		   Clear serious exception.  Works with tape drives which use the TMSCP tape controller interface

       clsub		   Clear subsystem.  Works with tape drives which use the TMSCP tape controller interface This command	is  restricted	to
			   root access only.

       eof, weof	   Write count end-of-file marks at the current position on the tape.

       eotdis		   Disable  end-of-tape  detection.   When the end of tape is reached, the tape will run off the reel.	Only the superuser
			   can issue this command.  The command remains in effect for the device until end-of-tape detection is enabled  with  the
			   eoten command.

       eoten		   Enable  end-of-tape	detection.   When the end-of-tape markers are reached, the tape is halted on the reel, between the
			   two end-of-tape markers.  Only the superuser can issue this command.  The command remains  in  effect  for  the  device
			   until end-of-tape detection is disabled with the eotdis command.  This is the default mode after a system boot.

       fsf		   Forward-space count files.

       fsr		   Forward-space count records.

       nocache		   Disables the use of the cache buffer for any tape drive that has the cache buffer feature.

       offline, rewoffl    Rewind the tape and place the tape unit off-line.

       retension	   Retensions the tape.  Retension means moving the tape one complete pass between EOT and BOT.

       rewind		   Rewind the tape.

       status		   Print status information about the tape unit.

Examples
       This example shows how to rewind the tape
       mt -f /dev/rmt0l rewind
       This example shows how to backspace the tape nmt1h three files:
       mt -f /dev/nrmt1h bsf 3
       This example shows how to write two end-of-file marks at the current position on tape nmt6h:
       mt -f /dev/nrmt6h eof 2

Return Values
       In  shell  scripts, returns a 0 exit status when the operation(s) were successful, 1 if the command was unrecognized, and 2 if an operation
       failed.

Files
       /dev/rmt?h or /dev/rmt?l
		 Raw magnetic tape interface with rewind when closed

       /dev/nmt?h or /dev/nmt?l
		 Raw magnetic tape interface with no rewind when closed

See Also
       dd(1), tar(1), ioctl(2), mtio(4), tms(4), environ(7)

																	     mt(1)
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