mc(7) Miscellaneous Information Manual mc(7)
NAME
mc - SCSI medium changer interface
DESCRIPTION
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 the System Administration Guide if you need to recreate device special files that are deleted acciden-
tally. 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 reser-
vation 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.
FILES
changer device special file header file for changer ioctl commands
RELATED INFORMATION
Commands: mcutil(1), mcicap(4), dsfmgr(8), scu(8), uerf(8)
Interfaces: op(7), tz(7), SCSI(7)
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mc(7)