mc(7)mc(7)NAMEmc - 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 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 reserva‐
tion 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
SEE ALSO
Commands: mcutil(1), mcicap(4), dsfmgr(8), scu(8), uerf(8)
Interfaces: op(7), tz(7), SCSI(7)mc(7)
