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)