mtio(4)mtio(4)Namemtio - magnetic tape interface
Description
The directory special files ``rmt0{a,l,m,h}, ..., rmt31{a,l,m,h''}
refer to the mass storage tape drives, that may exist on several dif‐
ferent buses depending on the bus/formatter/controller. On the BI, the
TMSCP controllers, are available. On the DSSI bus the TMSCP controller
is available. On the Q-bus the TMSCP controllers, and the TSV05 con‐
troller, are available. On the MASSBUS, there are the TM03, and TM78
formatters, On the UNIBUS, TS11 formatters, the TSU05 controller, and
the TMSCP controllers, are available. On VAXstation 2000s and MicroVAX
2000s, the TZK50 cartridge tape subsystem, is available. On the SCSI
bus, the SCSI tapes (see are available. The following description
applies to any mass storage tape drive.
For both the ``rewind'' and norewind special files, described later,
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 special files ``rmt0l, ..., rmt31l'' are low density, ``rmt0m, ...,
rmt31m'' are medium density (when a drive is ``triple density''), and
``rmt0h, ..., rmt31h'' are high density. All 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 example, the
TS11/TSU05/TSV05 supports only 1600 bpi so its rewind namespace is
``rmt0h, ..., rmt31h''.
For SCSI QIC tape devices, the special files ``rmt0a, ..., rmt31a'' are
QIC-24 density, ``rmt0l, ..., rmt31l'' are QIC-120 density, ``rmt0m,
..., rmt31m'' are QIC-150 density, and ``rmt0h, ..., rmt31h'' are
QIC-320 density (see tz 4 for further details.
The special files ``nrmt0{a,l,m,h}, ..., nrmt31{a,l,m,h}'' do not cause
a rewind when closed, regardless of density. When closed, the tape is
positioned between two tapemarks. The norewind namespace for the
TS11/TSU05/TSV05 example given above is ``nrmt0h, ..., nrmt31h''.
The rmt and nrmt special files are available to all ULTRIX utilities
that can perform I/O to tape. A number of magnetic tape ioctl opera‐
tions are available. The operations come under two ioctl request
groups. The MTIOCTOP ioctl is used to issue tape operation commands.
The MTIOCGET ioctl is used for getting status.
The mtop data structure defined in is passed as a parameter to the MTI‐
OCTOP ioctl. The mtop structure is:
struct mtop {
short mt_op;
daddr_t mt_count;
}
The mt_op field is used to specify the specific tape command to be per‐
formed. The mt_count field is used to specify the number of times the
command is to be performed (where applicable).
The following are tape operations supported in the MTIOCTOP ioctl.
These commands are specified in the mt_op field of the mtop structure.
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 for‐
ward the specified number of tape marks. (Tape marks
delimit files.) Upon successful completion of this com‐
mand, 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 begin‐
ning of the specified number of tape marks. Note that,
due to the difference in the side of a tape mark that a
reposition command leaves the tape positioned, the MTFSF
and MTBSF commands are not strictly reciprocal opera‐
tions. For example, if a tape is initially positioned
at the bottom 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. This error condition indi‐
cates that there were not as many records remaining in
the file as specified by the mt_count parameter.
MTBSR Repositions backward the number of records specified in
the mt_count field. This command returns a failure if a
tape mark is encountered. This error condition indi‐
cates that there were not as many records between the
present position and the beginning of the file as speci‐
fied in the mt_count parameter.
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 controller-based write-back caching.
Some tape controllers support caching as a performance
enhancement. Caching can speed up tape transfer opera‐
tions by keeping the unit streaming more effectively.
When using cached mode of operation, the MTFLUSH command
should be used to flush cached data to media. See the
description of MTFLUSH for more details.
MTNOCACHE Disables use of the controller's write-back cache. This
mode of operation can result in performance degradation
over cached mode.
MTCSE Clears serious exception. Certain operations cause the
tape unit to go into a serious exception state. An
example of this is when the physical end-of-media foil
is encountered. Typically, when a tape is in serious
exception state, all data transfer operations fail. In
order to acknowledge this exception condition and to
allow further operations to proceed, this command is
provided.
MTFLUSH Flushes the controller's write-back cache. This command
is intended to be used in conjunction with the MTCACHE
command. When caching has been enabled using the
MTCACHE command, writes to the tape will receive comple‐
tion status when the data has been transferred to the
controller's write-back cache. In the unlikely event of
controller error, it is possible that the data will not
be transferred to the physical media. To insure data
integrity, the MTFLUSH command is provided 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. 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. Retension means moving the tape
one complete pass between EOT and BOT.
The global variable errno is set to ENXIO if the command specified in
mt_op is not recognized or not supported by the respective tape driver.
Each or system call reads or writes the next record on the tape. In the
case of the record has the same length as the buffer given. During a
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. When n-buffered I/O is not being used (see 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 n-buffered I/O is being used (see a
zero byte count is returned when a tape mark is read, but another read
will not fetch the first record of the next tape file. In this situa‐
tion, all outstanding read requests return a status of 0. In order to
allow reading to proceed to the next file, the MTCSE command must be
issued. When a file open for writing is closed, two end-of-files (EOF)
are written. If a tape reaches the end-of-tape (EOT) marker, the ENOSPC
errno value is set.
Each or 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 hap‐
pens, any further or system calls fail with an error number of EINVAL.
This situation can only occur 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 system call to
reset the file offset to zero, for example, lseek(fd,0,0)
Restrictions
For SCSI tapes on VAX systems, the maximum tape record length is lim‐
ited to 16K bytes (K = 1024).
For SCSI tapes on both VAX and RISC systems, the MTCACHE, MTNOCACHE,
and MTFLUSH ioctls are not supported.
The MTRETEN ioctl is only supported by the SCSI QIC tape drive.
FilesSee Alsolseek(2), mu(4), scsi(4), stc(4), tms(4), ts(4), tu(4), tz(4),
MAKEDEV(8)mtio(4)
