mc - SCSI medium changer interface
The medium changer (mc) interface provides a means for
applications to control the robotic medium changers found
in tape and optical libraries or jukeboxes.
The mc driver may be used for any supported Small Computer
System Interface (SCSI) medium changer devices, and potentially
for other SCSI-compliant changer devices. Applications
use Unix I/O calls (open, close, ioctl) to access
changer devices, by means of device special files.
Changer device special files are typically created in the
/dev/changer directory by the dsfmgr utility on system
startup. Refer to the dsfmgr(8) Reference Page and System
Administration if you need to recreate device special
files that are deleted accidentally. The rz(7) and tz(7)
Reference Pages provide information on how device names
map to SCSI CAM lun addresses.
The format of a a medium changer device special file name
is:
/dev/changer/mcN
where N is an integer representing the instance of the
device that is assigned by dsfmgr at system startup.
The driver supports a number of ioctl commands that move
media in the library or return information about the
media. See the header file
/usr/sys/include/io/cam/mchanger.h for the ioctl commands
and their associated structs.
An application opens the device special file corresponding
to the changer device, executes appropriate ioctl commands,
then closes the device special file. Typically,
changer devices are not shared between applications, but
this is not due to any limitation on the changer or mc
driver, but rather to the possibility of confusing which
media belong to which application. An application on a
non-cluster system can assure that only it can use a
changer by opening that changer's device special file for
exclusive access, by including the O_EXCL flag in the open
call, and leaving the file open until the application is
completely done using the changer. However, if the
changer is on a shared bus in a cluster, it is possible
for an application on each cluster member to open the
device, even if each specifies O_EXCL, because that only
grants exclusive access on the local host. In this case
it may be useful for the application to use a SCSI device
reservation to assure exclusive access. (An ioctl command
is provided to facilitate reserving a changer.)
Refer to the Software Product Description for a list of
supported devices under the heading of SCSI CAM Layered
Components. Facilities are provided in the operating system
to allow the addition of some third-party SCSI-compliant
medium changers. Under Digital Unix V4.0 and later,
refer to the ddr_config(8) and ddr.dbase(4) reference
pages for instructions. Under Digital Unix V3.x, new
devices can be added to /sys/data/cam_data.c. See that
file for instructions.
changer device special file header file for changer ioctl
commands
Commands: mcutil(1), mcicap(4), dsfmgr(8), scu(8), uerf(8)
Interfaces: op(7), tz(7), SCSI(7)
mc(7)
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