mtio(7) Miscellaneous Information Manual mtio(7)
mtio - magnetic tape interface
This description applies to all mass storage tape drives. The /dev/tape directory special files, such as tape0, tape20_d0, tape3_d7,
refer to the mass storage tape drives. Tape drives can exist on different buses, and have different bus/formatter/controller dependencies.
This release supports revised device special file names for tape devices. Device file names have the base name tapeN, where N is a decimal
number denoting the instance of the device, and a suffix comprised of the characters _d followed by a single digit. For example tape0_d0.
This suffix determines the density of the tape device, according to the entry for the device in the /etc/ddr.dbase file.
There are also two forms of device special files that allow you to specify the defult density for a device and to enable compression, if
supported on a particular device: Device Special File Function
/dev/tape/tapeN default density for a rewind device
/dev/tape/tapeNc enable compression for a rewind device
/dev/ntape/tapeN default density for a non-rewind device
/dev/ntape/tapeNc enable compression for a non-rewind device
Note that with the new device special file naming, there is a direct mapping from the old name suffix to the new name suffix as follows:
Old Suffix New suffix
l (low) _d0
m (medium) _d2
h (high) _d1
a (alternate) _d3 There are two sets of device names for tape that both conform to the new naming convention. The /dev/tape
directory for rewind devices and the /dev/ntape directory (for no rewind). To determine which device special file to use, you can look in
the /etc/ddr.dbase file.
The special files in /dev/tape cause a loaded and on-line tape to automatically rewind to the beginning-of-tape (BOT) when closed. Low,
medium, and high density are relative to the densities supported on a particular tape drive. See tz(7) for more information. The special
files in /dev/ntape do not cause a rewind when closed, regardless of density. The tape remains in the same position as it was when it was
These special files are available to all operating system utilities that can perform I/O to tape.
Magnetic Tape Operations
Magnetic tape ioctl system calls perform tape operations. The operations come under three ioctl request groups. The MTIOCTOP ioctl is
used to issue tape operation commands. The MTIOCGET ioctl is used to obtain status. The MTIOCRDPOS ioctl is used to obtain tape position
information from the tape drive.
MTIOCTOP ioctl Commands
The mtop data structure is passed as a parameter to the MTIOCTOP ioctl. Please see the /usr/include/sys/mtio.h header file for a defini-
tion of the mtop structure.
The mt_op field specifies the MTIOCTOP tape command to be performed. The mt_count field specifies the number of times the command should
be performed (where applicable).
The following MTIOCTOP tape commands are supported: Writes an end-of-file to the tape. Physically, an end of file consists of a tape mark.
Repositions forward the number of files specified in the mt_count field. This command repositions the tape forward the specified number of
tape marks. (Tape marks delimit files.) Upon successful completion of this command, the tape is physically positioned at the end of the
specified number of tape marks. Repositions backward the number of files specified in the mt_count field. This command repositions the
tape backward the specified number of tape marks. (Tape marks delimit files.) Upon successful completion of the command, the tape is
physically positioned at the beginning of the specified number of tape marks.
Because MTFSF leaves the tape positioned at the end of a tape mark and MTBSF leaves it positioned at the beginning, these two commands are
not strictly reciprocal operations. For example, if a tape is initially positioned at the beginning of tape (BOT) and the command MTFSF 1
is issued followed by the command MTBSF 1, the tape does not return to the BOT position. Instead, the tape is positioned on the BOT side
of the first tape mark.
Repositions forward the number of records specified in the mt_count field. This command returns a failure if a tape mark is encountered to
indicate that there were not as many records remaining in the file as there were records specified by the mt_count field. Repositions
backward the number of records specified in the mt_count field. This command returns a failure if a tape mark is encountered to indicate
that there were not as many records between the present position and the beginning of the file as specified in the mt_count field. Rewinds
the tape. This command repositions to the beginning of the tape. Rewinds and unloads the tape. Does not perform any tape operation.
Always returns success from a tape file. Enables the use of hardware-based write-back caching for better performance. Caching can speed
tape transfer operations, thereby keeping the unit streaming more effectively. If you use the MTCACHE command, use the MTFLUSH command to
flush cached data to media. SCSI tape drives that support caching have their cache turned on automatically by the driver; the MTCACHE com-
mand is unnecessary. Disables use of the controller's hardware-based write-back cache. Tape operations using this command are slower than
tape operations using the MTCACHE command, but do not require use of the MTFLUSH command to guarantee that the data is immediately written
to tape. Clears a serious exception. Certain operations cause the tape unit to go into a serious exception state. This can happen, for
example, when the physical end-of-media foil is encountered. Typically, when a tape is in a serious exception state, all data transfer
operations fail. Use the MTCSE command to acknowledge the exception condition and allow further operations to proceed. Clears a hardware
or software error. Clears the subsystem. Enables 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 MTENAEOT command remains in effect for the
device until end-of-tape detection is disabled with the MTDISEOT command. This is the default mode after a system boot. Disables 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 MTDISEOT
command remains in effect for the device until end-of-tape detection is enabled with the MTENAEOT command. Flushes the hardware-based
write-back cache. For tapes that have controller-based caching (for example, TMSCP tapes), use this command with the MTCACHE command. For
tapes that have device-based caching (for example, SCSI tapes), use this command by itself. When caching has been enabled, writes to the
tape receive a completion status when the data has been transferred to the cache, not when the data is transferred to the media. Use the
MTFLUSH command to force a flush of the cache to physical media. Failure of this command with errno set to ENXIO means that the drive does
not support the flush command. However, SCSI devices do not return ENXIO; therefore you cannot rely on the MTFLUSH command to determine
whether caching is enabled. Failure with errno set to EIO indicates that the cache flush has failed. In this case, the application should
retry writing records that have been written since the last successful MTFLUSH command. Retensions the tape by moving the tape one com-
plete pass between EOT and BOT. Moves the tape to the end of recorded data. Erases the tape. Loads and rewinds the tape. Loads the
tape. Unloads the tape. Enables scatter/gather IO for the readv() and writev() system calls. After this command, any readv() or writev()
system calls will cause the tape driver to transfer all iovec buffers in the list in a single transfer to tape. Disables scatter/gather IO
for the readv() and writev() system calls. After this command, each buffer provided in a readv() or writev() system call will be trans-
ferred by itself. Sends a SCSI LOCATE command to the tape drive, telling it to position the tape to the SCSI logical block address speci-
fied by the mt_count field. Sends a SCSI LOCATE command to the tape drive, telling it to position the tape to the device specific address
specified by the mt_count field.
MTIOCGET ioctl Requests
The mtget data structure is passed as a parameter to the MTIOCGET ioctl. Please see the /usr/include/sys/mtio.h header file for a defini-
tion of the mtget structure.
The following list describes the fields of the mtget data structure: Provides driver-specific drive status information. This is the same
information provided in the stat member of the devget structure used by the DEVIOCGET ioctl.
For the TMSCP driver, please see the /usr/include/sys/devio.h> header file for bit definitions of the stat member.
For the SCSI tape driver, please see the /usr/include/io/cam/cam_tape.h header file for bit definitions of the ts_flags member of
the TAPE_SPECIFIC structure. Provides driver-specific error information.
For the TMSCP driver, please see table B-1 in the /usr/include/io/dec/sysap/mscp_msg.h header file for error code definitions.
For the SCSI tape driver, the mt_erreg member contains the sense key byte of the sense data from a SCSI REQUEST SENSE command.
Please see the /usr/include/io/cam/scsi_all.h header file or the SCSI-2 standard for definitions of the sense key. Also included in
this byte are the Filemark, EOM, and ILI bits as defined in the SCSI-2 standard. Contains command-specific results. For example,
after a read command using a variable block-length tape, mt_resid contains the residual number of bytes not transferred. Another
example is the space command. After a space command, mt_resid contains the number of blocks or files not spaced over. Contains the
file position of the tape. Contains the record position within a file.
Extended error information can be found in the /usr/include/io/common/deveei.h header file.
MTIOCRDPOS ioctl Requests
The mtrdpos data structure is passed as a parameter to the MTIOCRDPOS ioctl. Please see the /usr/include/sys/mtio.h header file for a def-
inition of the mtrdpos, mtrdposs, and mtrdposl structures.
The following two structure members of the mtrdpos structure are information provided by the application to control the format of the data
returned by the tape drive. The un member of this structure contains data passed back to the application from the tape drive.
This is a single bit which, if set, tells the SCSI tape driver to send the READ POSITION command, requesting that the tape drive return the
long data format. This is a single bit which, if set, tells the SCSI tape driver to send the READ POSITION command, requesting that the
tape drive return the short data format with device specific addresses, rather than the SCSI logical block addresses. If the long_format
field is set, then this bit is ignored by the tape driver. This is a union of the short (mtrdposs) and the long (mtrdposl) data format
structures. The values in the members of these data structures are copies of the information in the short and long data formats returned
by the SCSI READ POSITION command as specified in the SCSI standard documentation. Please note that the first_blk member of the mtrdposs
structure and the blk_num member of the mtrdposl structure return information in the form which may be used with the MTSEEK and MTSEEKDS
commands of the MTIOCTOP ioctl.
System Call Behavior
Each read() or write() system call reads or writes the next record on the tape. In the case of a write() system call, the record has the
same length as the buffer given. During a read() system call, the record size is passed back as the number of bytes read, provided it is
no greater than the buffer size. If the record is long, an error is returned. Seeks are ignored. Positioning is done with a tape ioctl
call. A zero byte count is returned when a tape mark is read, but another read fetches the first record of the next tape file. When a
file open for writing is closed and the last user command was a write, two end-of-file marks (EOF) are written. If a tape reaches the end-
of-tape (EOT) marker, the ENOSPC errno value is set.
Each read() or write() system call causes the file offset associated with the device special file to be incremented. This file offset is
reset to 0 when the file is closed. If a program does an unusually large number or reads and writes to the tape, it is possible to cause
the file offset to be incremented beyond the maximum allowable value. When this happens, any further read() or write() system calls fail
with an error number of EINVAL. This situation can occur only if the tape is read or written to several times over, using repositioning
commands such as MTREW to reposition backwards on the tape. It is recommended that any application which expects to make numerous passes
over the tape use the lseek() system call to reset the file offset to zero, for example, lseek(fd,0,0).
The MTRETEN ioctl is supported only by the SCSI QIC tape drive.
For the DEVIOCGET and MTIOCGET ioctls, the DEV_TPMARK, DEV_SHRTREC, DEV_EOM, DEV_OFFLINE, DEV_SOFTERR, and DEV_HARDERR tape driver flags
are in an indeterminate state unless the application has gotten an unexpected return value from a system call (that is, read x bytes from
tape, and if the return value does not equal x, then the flags are not in an indeterminate state).
The MTIOCGET ioctl call is non-intrusive. This means that the device driver implements support for MTIOCGET solely by interrogating its
data structures. The device should not be perturbed. This also means that the MTIOCGET ioctl call always returns ESUCCESS. However,
because the implementation of the MTIOCGET ioctl is dependent upon each device driver, and upon each device driver's ioctl code, an errno
status is sometimes returned. An errno status returned from a MTIOCGET call indicates an error condition inside the driver itself. These
are usually pre-processing errors inside the device driver's ioctl code.
The MTIOCTOP ioctl commands set errno to [ENXIO] if the command specified in mt_op is not recognized or not supported by the respective
The following list describes the possible errno status returned from MTIOCGET: The operation was successful. An error occurred while try-
ing to copy out ioctl data (FOP_IOCTL). The device database structure was not present (no nexus). The device-specific data structure was
not present (device not opened). The device driver data structure was not present (device not opened). The request was invalid or the
requested function is not supported. The requested function is not supported.
lseek(2), SCSI(7), tz(7), MAKEDEV(8) delim off