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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XPT(4) BSD Kernel Interfaces Manual XPT(4)
NAME
xpt -- CAM transport layer interface
SYNOPSIS
None.
DESCRIPTION
The xpt driver provides a way for userland applications to issue certain CAM CCBs to the kernel.
Since the xpt driver allows direct access to the CAM subsystem, system administrators should exercise caution when granting access to this
driver. If used improperly, this driver can allow userland applications to crash a machine or cause data loss.
KERNEL CONFIGURATION
There is no kernel configuration required for the xpt driver. It is enabled when SCSI support is enabled in the kernel. There is one
instance of the xpt driver per CAM transport layer instance. Since there is currently only one CAM transport layer, there will only be one
instance of this driver.
IOCTLS
CAMIOCOMMAND This ioctl takes certain kinds of CAM CCBs and passes them through to the CAM transport layer for action. Only the following
CCB types are supported:
XPT_SCAN_BUS
XPT_RESET_BUS
XPT_SCAN_LUN
XPT_ENG_INQ
XPT_ENG_EXEC
XPT_DEBUG
XPT_DEV_MATCH
XPT_PATH_INQ
The above CCBs are the only ones supported since it makes more sense to send them through a generic passthrough device rather
than a passthrough device tied to a particular underlying SCSI device.
CAMGETPASSTHRU This ioctl takes an XPT_GDEVLIST CCB, and returns the passthrough device corresponding to the device in question.
FILES
/dev/xpt0 Character device node for the xpt driver.
DIAGNOSTICS
None.
SEE ALSO
cam(3), cam_cdbparse(3), pass(4), camcontrol(8)
HISTORY
The CAM transport layer driver first appeared in FreeBSD 3.0.
AUTHORS
Kenneth Merry <ken@FreeBSD.org>
BSD
October 10, 1998 BSD