HPUX mt[7]
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mt.1
mt(7) mt(7)
NAME
mt - magnetic tape interface and controls
DESCRIPTION
This entry describes the behavior of HP magnetic tape interfaces and
controls, including DDS and QIC cartridge drives. The files
/dev/rmt/* refer to specific raw tape drives, and the behavior of each
given unit is specified in the major and minor numbers of the device
special file.
The following naming conventions are recommended because they relate
most of the mode flags to the device name:
/dev/rmt/(c#d)#[hml]{c}{n}{b}
or
/dev/rmt/(c#d)#qic[525|150|120]{n}{b}
In this format, c#d indicates the controller number (optionally
specified by the system administrator), # is the device number, hml
indicates the density (h (high) for 6250 bpi, m (medium) for 1600 bpi,
and l (low) for 800 bpi), c indicates data compression, n indicates no
rewind on close and b indicates Berkeley style. For example,
/dev/rmt/2mn is device lu 2, AT&T style at 1600 bpi with no rewind and
no compression. The selection of controller and unit numbers is
system dependent, and is discussed in the appropriate system
administrator's manual.
For S800 QIC devices, qic (without a format number, i.e. default
format) indicates the best capacity format for the drive and currently
loaded medium, qic525 for QIC-525/320 format, qic150 for QIC-150
format, and qic120 for QIC-120 format.
Accessing a QIC device through a /dev/rmt/(c#d)#[hml] device file is
equivalent to /dev/rmt/(c#d)#qic in that a default format will be
selected.
The operation of a tape drive is controlled by mode flags, which are
usually encoded as bits in the minor number of the device special
file.
no-rewind Unless this mode is requested, the tape is
automatically rewound upon close. When a rewind
on close is not desired, the n flag should be used
in the device name.
style When this mode is requested, the tape drive
behaves as on Berkeley systems; when not
requested, the drive behaves as on AT&T UNIX
operating systems. The details are described
below. The ioctl(2) operations described below
work in both modes. The mt(1) tape movement
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utility requires that the Berkeley mode be
specified.
density This may be used to select the density of the tape
being written. Values that may be selected include
6250, 1600, and 800 bpi, depending on the
capabilities of the specific tape drive. This
corresponds to the h, m and l flags in the
recommended device name. For DDS (digital audio
tape) and QIC (quarter inch tape) format devices,
density designations are not used.
format This may be used to select the QIC format of the
cartridge being written. See DEPENDENCIES.
compression On tape drives that support data compression,
selecting the device file with c causes the data
to be written or read in compressed mode.
Refer to the system administrator manual for your computer for more
specific details of how to select the modes for a given device.
When opened for reading or writing, the tape is assumed to be
positioned as desired.
When a file opened for writing is closed, two consecutive EOF marks
are written if, and only if, one or more writes to the file have
occurred. The tape is rewound unless the no-rewind mode has been
specified, in which case the tape is positioned before the second EOF
just written. For QIC devices only one EOF mark is written and the
tape is positioned after the EOF mark if the no-rewind mode has been
specified.
When a file open for reading only is closed and the no-rewind bit is
not set, the tape is rewound. If the no-rewind bit is set, the
behavior depends on the style mode. For AT&T-style devices, the tape
is positioned after the EOF following the data just read. For
Berkeley-style devices, the tape is not repositioned in any way.
Each read(2) or write(2) call reads or writes the next record on the
tape. For writes, the record has the same length as the buffer given
(within the limits of the hardware).
During a read, the record size is passed back as the number of bytes
read, up to the buffer size specified. The number of bytes ignored
(for records longer than the buffer size specified) is available in
the mt_resid field of the mtget structure via the MTIOCGET call of
ioctl(2). The buffer and size might have implementation-dependent
alignment restrictions.
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Reading an EOF mark is returned as a successful zero-length read; that
is, the data count returned is zero and the tape is positioned after
the EOF, enabling the next read to return the next record.
DDS format devices also support setmarks which are hierarchically
superior to filemarks. A setmark is used to delineate a group (set)
of files. Reading a setmark is also returned as a zero-length read.
The two can be distinguished by unique bits in the mt_gstat field.
Spacing operations (back or forward space, setmark, file or record)
leave the tape positioned past the object being spaced to in the
direction of motion. In other words, backspacing a file leaves the
the tape positioned before the file mark; forward spacing a file
leaves the tape positioned after the file mark. This is consistent
with all classical usage on tapes.
Seeks on a magnetic tape device are ignored. Instead, the ioctl(2)
operations below can be used to position the tape and determine its
status.
The header file <sys/mtio.h> has useful information for tape handling.
The following is included from <sys/mtio.h> and describes the possible
tape operations:
/* mag tape I/O control requests */
l l l.
#define MTIOCTOP _IOW(m,1,struct mtop) /* do mag tape op */
#define MTIOCGET _IOR(m,2,struct mtget) /* get tape status */
/* structure for MTIOCTOP - mag tape op request */
struct mtop {
l l l l l.
short mt_op; /* operations defined below */
daddr_t mt_count; /* how many of them */
};
/* operations */
l l l.
#define MTWEOF 0 /* write filemark (end-of-file record) */
#define MTFSF 1 /* forward space file */
#define MTBSF 2 /* backward space file */
#define MTFSR 3 /* forward space record */
#define MTBSR 4 /* backward space record */
#define MTREW 5 /* rewind */
#define MTOFFL 6 /* rewind, put drive offline */
#define MTNOP 7 /* no-op, may set status */
#define MTEOD 8 /* DDS and QIC only. seek to end-of-data */
#define MTWSS 9 /* DDS only. write setmark(s) */
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#define MTFSS 10 /* DDS only. space forward setmark(s)*/
#define MTBSS 11 /* DDS only. space backward setmark(s)*/
/* structure for MTIOCGET - mag tape get status command */
struct mtget {
l l l l.
long mt_type; /* type of magtape dev. */
long mt_resid; /* residual count */
/* The following two registers are device dependent */
l l l l.
long mt_dsreg1; /* status register (msb) */
long mt_dsreg2; /* status register (lsb) */
/* The following is a device-independent status word */
l l l l.
long mt_gstat; /*generic status*/
long mt_erreg; /* error register */
Information for decoding the mt_type field can be found in <sys/mtio.h>.
EXAMPLES
Assume that fd is a valid file descriptor. The first example writes
two consecutive filemarks on the tape:
#include <sys/types.h>
#include <sys/mtio.h>
struct mtop mtop;
mtop.mt_op = MTWEOF;
mtop.mt_count = 2;
ioctl(fd, MTIOCTOP, &mtop);
If fd is a valid file descriptor for an open DDS drive, the following
example spaces forward to just past the next setmark:
#include <sys/types.h>
#include <sys/mtio.h>
struct mtop mtop;
mtop.mt_op = MTFSS;
mtop.mt_count = 1;
ioctl(fd, MTIOCTOP, &mtop);
Now suppose that fd is a valid file descriptor for an opened tape
device, and suppose further that it has just returned zero from a
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read(2) request. To verify that the tape has just read a filemark,
the application could issue the following system call:
#include <sys/types.h>
#include <sys/mtio.h>
struct mtget mtget;
ioctl(fd, MTIOCGET, &mtget);
if (GMT_EOF(mtget.mt_gstat)) {
/* code for filemark detection */
}
WARNINGS
It is impossible to write a program that leaves a tape positioned at
the beginning of the tape on an AT&T-style device with the no-rewind
bit set because closing the device file upon the program's termination
repositions the tape after the first EOF mark.
An AT&T-style device file opened for writing to blank media may cause
an error condition at close (due to attempting to space to the non-
existent next EOF mark) unless a tape alteration operation has been
performed.
HP-UX silently enforces a tape record blocking factor (MAXPHYS) on
large I/O requests. For example, a user write request with a length
of ten times MAXPHYS will actually reach the media as ten separate
records. A subsequent read (with ten times MAXPHYS as a length) will
look like a single operation to the user, even though HP-UX has broken
it up into ten separate read requests to the driver. Such activity is
normally transparent to the user unless:
o The user picks an arbitrary read length that is greater than
MAXPHYS.
o The user attempts to read a third-party tape containing
records larger than MAXPHYS.
Since the value for MAXPHYS is relatively large (usually >= 64K
bytes), this is typically not a problem.
Write operations on a QIC device can be initiated only at BOT or EOD.
No overwriting is allowed by positioning the tape in the middle of
recorded data.
The offline operation puts the QIC drive offline. The cartridge is
not ejected as done for DDS. To put the drive back online, the
cartridge has to be manually ejected and then reinserted.
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Sequential-Access devices that use the SCSI-1 I/O interface do not
always report true media position.
DEPENDENCIES
Series 300/400
QIC is not supported.
Series 800
The MTNOP operation does not set the device-independent status
word.
QIC devices do not always report media position accurately.
If no QIC specific format is specified in the minor number, the
best capacity format for the drive and currently loaded medium is
used.
The maximum I/O request for QIC devices is limited to 64K - 1
(65535) bytes.
Efficient use of streaming tape drives with large internal
buffers and immediate-reporting require the following end-of-tape
procedures:
All writes near the EOT foil (which is not on the recording
surface) complete without error if actually written to the tape.
Once the tape drive determines that the foil has been passed,
subsequent writes do not occur and an error message is returned.
Since some applications require that a trailer be written for
multiple tape operations, a user request for magnetic tape status
that reflects the EOT condition signals the driver to drop all
write barriers. Caution must be exercised to keep the tape on
the reel.
When reading near the end-of-tape, the user is not informed of
the EOT foil marker. Instead, the typical double EOF marks or a
pre-arranged trailer signals the logical end-of-tape.
The EOT description above applies in the default case when
immediate-reporting mode is allowed by a value encoded in the
minor number. When not permitted by the minor number, the EOT
operation attempts to emulate compatibility-mode on other HP-UX
machines. In this mode, the write encountering the EOT foil
returns an error with the tape automatically backing up over that
record. The read encountering the EOT foil returns an error.
Since magnetic tape drives vary in EOT sensing due to differences
in the physical placement of sensors, any application (such as
multiple-tape cpio(1) backups) requiring that data be continued
from the EOT area of one tape to another tape must be restricted.
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Therefore, the tape drive type and mode should be identical for
the creation and reading of the tapes.
The following macros are defined in <sys/mtio.h> for decoding the
generic status of the tape drive (returned in the mt_gstat
field):
l l. GMT_BOT(x) /* At beginning of tape */ GMT_EOD(x)
/* DDS and QIC End-of-Data encountered */ GMT_EOF(x) /* At
an EOF mark */ GMT_EOT(x) /* At end of tape */
GMT_DR_OPEN(x) /* Drive door is open */ GMT_IM_REP_EN(x) /*
Immediate reporting mode enabled */ GMT_ONLINE(x) /* Drive is
on line */ GMT_SM(x) /* setmark encountered */ GMT_WR_PROT(x)
/* Tape is write protected */ GMT_D_6250(x) /* Density is 6520
bpi */ GMT_D_1600(x) /* Density is 1600 bpi */ GMT_D_800(x)
/* Density is 800 bpi */ GMT_COMPRESS(x) /* Data compression
enabled */ GMT_QIC_FORMAT(x) /* QIC format on tape */
GMT_QIC_MEDIUM(x) /* QIC medium type*/
If GMT_IM_REP_EN(x) is true, the drive reports completion of each
operation immediately after receiving it.
AUTHOR
mt was developed by HP and the University of California, Berkeley.
FILES
/dev/rmt/*
SEE ALSO
dd(1), mt(1), ioctl(2), ct(7).
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