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mtio(7)
NAME
mtio - magnetic tape interface
DESCRIPTION
This description applies to all mass storage tape drives. The /dev
directory special files rmt0{a,l,m,h},...,rmt31{a,l,m,h} refer to the mass
storage tape drives. Tape drives can exist on different buses, and have
different bus/formatter/controller dependencies.
Special Files
For both the rewind and norewind special files, the unit number represents
a symbolic count that has no connection with the actual plug or controller
number of a particular tape drive. As each tape unit special file is
created, the number counts up from 0 to 31 for a total of 32 tape drives.
The rmt0l,...,rmt31l special files are low density, the rmt0m,...,rmt31m
special files are medium density (also known as triple density), and the
rmt0h,...,rmt31h special files are high density. These special files 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.
For SCSI QIC tape devices, the rmt0a,...,rmt31a special files are QIC-24
density, the rmt0l,...,rmt31l special files are QIC-120 density, the
rmt0m,...,rmt31m special files are QIC-150 density, and the
rmt0h,...,rmt31h special files are QIC-320 density. See tz(7) for more
information.
The nrmt0{a,l,m,h},...,nrmt31{a,l,m,h} special files do not cause a rewind
when closed, regardless of density. When closed, the tape is positioned
between two tapemarks. The norewind namespace for the QIC-24 example shown
previously is nrmt0a,...,nrmt31a The rmt and nrmt 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 two ioctl request groups. The MTIOCTOP ioctl is used to issue
tape operation commands. The MTIOCGET ioctl is used to obtain status.
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 definition 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:
MTWEOF Writes an end-of-file to the tape. Physically, an end of file
consists of a tape mark.
MTFSF 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.
MTBSF 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.
Note
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.
MTFSR 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.
MTBSR 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.
MTREW Rewinds the tape. This command repositions to the beginning of the
tape.
MTOFFL Rewinds and unloads the tape.
MTNOP Does not perform any tape operation. Always returns success from a
tape file.
MTCACHE
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 command is unnecessary.
MTNOCACHE
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.
MTCSE 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.
MTCLX Clears a hardware or software error.
MTCLS Clears the subsystem.
MTENAEOT
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.
MTDISEOT
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.
MTFLUSH
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.
MTRETEN
Retensions the tape by moving the tape one complete pass between EOT
and BOT.
MTSEOD Moves the tape to the end of recorded data.
MTERASE
Erases the tape.
MTONLINE
Loads and rewinds the tape.
MTLOAD Loads the tape.
MTUNLOAD
Unloads the tape.
MTENASGIO
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.
MTDISSGIO
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 transferred by itself.
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 definition of the
mtget structure.
The following list describes the fields of the mtget data structure:
mt_dsreg
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.
mt_erreg
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.
mt_resid
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.
mt_fileno
Contains the file position of the tape.
mt_blkno
Contains the record position within a file.
Extended error information can be found in the
/usr/include/io/common/deveei.h header file.
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).
RESTRICTIONS
The MTRETEN ioctl is supported only by the SCSI QIC tape drive.
RETURN VALUES
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.
ERRORS
The MTIOCTOP ioctl commands set errno to [ENXIO] if the command specified
in mt_op is not recognized or not supported by the respective tape driver.
The following list describes the possible errno status returned from
MTIOCGET:
[ESUCCESS]
The operation was successful.
[EFAULT]
An error occurred while trying to copy out ioctl data (FOP_IOCTL).
[ENODEV]
The device database structure was not present (no nexus).
[ENXIO]
The device-specific data structure was not present (device not
opened).
[ENOMEM]
The device driver data structure was not present (device not
opened).
[EINVAL]
The request was invalid or the requested function is not supported.
[ENOTSUP]
The requested function is not supported.
FILES
/dev/rmt???
/dev/nrmt???
RELATED INFORMATION
lseek(2), SCSI(7), tz(7), MAKEDEV(8)