mtio(7)mtio(7)NAMEmtio - magnetic tape interface
DESCRIPTIONmtio describes the generic interface provided for dealing with the
various types of magnetic tape drives supported on Silicon Graphics
systems. 1/4" (QIC) cartridge tapes, 1/2" nine-track tapes, 8 mm video
tapes, IBM 3590, STK 9490, STK SD3, Sony GY-10 and GY-2120, Ampex DST-
310, DAT (digital audio tapes), and DLT are currently supported. (Not
all systems support all tapedrives.)
Tape drives are accessed through special device files in the /dev/mt/*
and /dev/rmt/* directories. The mt directory is a link to the rmt
directory for ease of porting; the block interface sometimes associated
with the devices in mt is not supported under IRIX. Refer to intro(7)
for a general description of the naming conventions for device files in
these subdirectories. Naming conventions for the specific devices are
listed under tps(7M).
Normally the device specific name is linked to a user-friendly synonym
for ease of use. Many commands that manipulate magnetic tape refer to
these synonyms rather than device specific names. There are up to four
user-friendly device synonyms:
/dev/tape This is the tape unit that is the default system tape
drive. It is linked to one of the specific device names
in /dev/rmt. This device rewinds the tape when closed.
For QIC tapes, the devices linked to /dev/tape do
software byte swapping to be compatible with the IRIS
2000 and 3000 series systems; the non-byte swapping
device is also available, and should almost always be
used for all tape types other than QIC.
/dev/nrtape Same as /dev/tape, except the tape is not rewound when
closed.
/dev/tapens Same as /dev/tape, except no byte swapping is done;
normally created only for QIC tapes.
/dev/nrtapens Same as /dev/nrtape, except no byte swapping is done;
normally created only for QIC tapes.
See the (unfortunately somewhat confusing) script /dev/MAKEDEV for
details of which devices are linked to /dev/tape for each tape type. In
particular, look at the tapelinks target for the default links. Also be
aware that if a /dev/tape exists as a link to a valid tape device, it is
left as is, in order to preserve local changes.
Note that even the norewind tape devices can be rewound by the system, in
some cases. In particular, all tapes are rewound on first use after a
system boot, and when detected by the driver, are rewound after the tape
has been changed. DAT drives are also rewound when the drive is switched
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mtio(7)mtio(7)
between audio and data modes. This means that if you wish to append a
new dataset to a tape that already contains datasets, you should always
issue the mt feom command AFTER loading the tape, just prior to using the
program that will append to the tape. Do not count on a tape remaining
at EOD, just because that is where it was before it was removed.
The system makes it possible to treat the tape similar to any other file,
with some restrictions on data alignment and request sizes. Seeks do not
have their usual meaning (that is, they are ignored) and it is not
possible to read or write other than a multiple of the fixed block size
when using the fixed block device. Writing in very small blocks (less
than 4096 bytes) is inadvisable because this tends to consume more tape
(create large record gaps verses data for 9-track and may pad to a device
specific boundary for others, such as 8mm, if the drive isn't kept
streaming); it may also cause the tape to stop its streaming motion,
increasing wear on the drive and decreasing throughput.
The standard QIC tape consists of a series of 512-byte records terminated
by an end-of-file. Other tape devices (such as 9-track, 8 mm, and DAT)
typically support both fixed size block format and variable size blocks
format.
When using the variable format, there is an upper limit to the size of a
single read or write, typically the size of the RAM buffer on the drive.
At this time, the upper limit is 64K bytes on 9-track and 240K bytes on
the 8 mm drives. This information can be obtained by use of the
MTIOCGETBLKINFO ioctl (for SCSI tape drives only). The main use of this
format is to allow small header blocks at the beginning of a tape file,
while the rest are typically the same (larger) size.
When the fixed block size device is used, the size of a single read or
write request is limited only by physical memory. Currently the default
size is 1024 bytes on 8mm, and 512 bytes for all others. This size can
be reset with the MTSCSI_SETFIXED ioctl or the mt setblksz XXX command;
the value remains set until the next boot or reset via ioctl. If the
variable blocksize device is used, the block size reverts to the default
on the next use of the fixed blocksize device. The default fixed
blocksize for the tps driver is set from the table in the
/var/sysgen/master.d/scsi file, as are the drive types, based on the
inquiry data returned by the drive.
A tape is normally open for reading and/or writing, but a tape cannot be
read and written simultaneously. After a rewind, a space, an unload, or
an MTAFILE ioctl, writes can follow reads and vice-versa, otherwise only
reads, or only writes can be done unless the tape is first closed;
performing an MTWEOF ioctl is considered to be a write operation in this
sense; after reading, an MTWEOF is not allowed unless one of the
operations above is first executed, or the tapedrive is closed and
reopened.
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mtio(7)mtio(7)
Whenever the tape is closed after being written to, a file-mark is
written (2 on 9-track tapes) unless the tape has been unloaded or rewound
just prior to the close; if the last operation before the close is an
MTWEOF, no additional filemarks are written at close.
Some tape drives support more than one speed; for SCSI 9-track tape
drives, the default is 100 ips (streaming mode); this can be set to 50
ips by using the MTSCSI_SPEED ioctl. Some tape drives such as the
Kennedy 96XX models do auto density select when reading; this can be
disabled only by using the drive's front panel setup mode.
The MTSCSIINQ ioctl returns the inquiry data for SCSI tape drives. The
buffer supplied in the 3rd argument must be at least MTSCSI_INQUIRY_LEN
bytes long. It returns all the information returned by the inquiry
command, with the identification string starting at offset 8 in the
buffer.
The MTSCSI_SENSE ioctl returns the most recent request sense data for
SCSI tape drives. The buffer must be at least MTSCSI_SENSE_LEN long,
although less data may be returned. The data is from the most recent
request sense SCSI command issued, and may not be directly related to the
most recent system call to the tape driver, since at open, and for some
ioctl's, multiple SCSI commands are issued. The buffer is never cleared.
The data is as returned by the tape drive, and the returned data must be
examined to return the valid length. Currently at most 64 bytes will be
returned, but callers must not assume this will remain true in the
future.
The MTANSI ioctl allows writing of ANSI 3.27 style labels after the early
warning point (naturally, drives that don't support variable sized blocks
won't support 80 byte labels). It is currently implemented only for SCSI
tape drives. It remains set until the next close, or reset with a 0
argument. If used, standard Silicon Graphics EOT handling for tar, bru,
and cpio won't work correctly while set. An arg of 1 enables, 0
disables. NOTE: When the EOT marker is encountered, the current I/O
operation returns either a short count (if any I/O completed), or -1 (if
no I/O completed); it is the programmers responsibility to determine if
EOT has been encountered by using MTIOCGET or logic internal to the
program. This ioctl should be issued AFTER encountering EOT, if ANSI
3.27 EOT handling is desired. Subsequent I/Os are then allowed and
return the count actually transferred or -1 if the drive was unable to
transfer any data. The standard calls for writing a FM before the label.
If this is not done, the drive may return label info as data.
The MTBUFFMMODE ioctl can be used to enable/disable the writing of
buffered filemarks. An arg of 1 enables, 0 disables. Normally, commands
which result in the writing of filemarks will not complete until write
data and subsequent filemarks are actually written to tape. By enabling
buffered filemark writing, on those devices which support it, one can
achieve higher performance.
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The MTGETATTR ioctl can be used to obtain functional attributes of a tape
device. The arg will be a pointer to an instance of the structure
mt_attr. (See /usr/include/sys/mtio.h) The attribute whose value is to be
determined must be specified in ma_name, space for the return value must
be allocated, and a pointer to it and its size must be specified in
ma_value and ma_vlen respectively. Currently available attributes are
MT_ATTR_NAME_REWIND
MT_ATTR_VALUE_TRUE or MT_ATTR_VALUE_FALSE is returned depending on
whether the device is a rewind-on-close device.
MT_ATTR_NAME_VARIABLE
MT_ATTR_VALUE_TRUE or MT_ATTR_VALUE_FALSE is returned depending on
whether the device is a variable block device.
MT_ATTR_NAME_SWAP
MT_ATTR_VALUE_TRUE or MT_ATTR_VALUE_FALSE is returned depending on
whether the device is a swap device.
MT_ATTR_NAME_COMPRESS_DENS
For devices which don't support multiple densities but do support a
compression mode, "compress" or "uncompress" is returned depending
on whether the device is a compression device. For devices which do
support multiple densities, the device specific density/compression
value is returned.
MT_ATTR_NAME_DEVSCSI
The full pathname of the associated devscsi device is returned.
The MTFPMESSAGE ioctl can be used to display an ASCII message on the
device's diagnostic panel, if the device supports such a feature. The arg
will be a pointer to an instance of the structure mt_fpmsg. (See
/usr/include/sys/mtio.h) The ASCII message and it's length will be
specified in mm_msg and mm_mlen respectively.
Tape devices with multiple initiators may have a persistent reservation
placed on them with the MTRSV op code of the MTIOCTOP ioctl. If another
system has already placed a reservation against this device, the ioctl
will return EBUSY. The reservation may be cleared with the MTRLS op code
of the MTIOCTOP ioctl. Both the MTRSV and MTRLS operations are
restricted to users with CAP_DEVICE_MGT capability.
Two flags, returned in the mt_dposn field of the mtget struct after a
call to MTIOCGET, are provided for the purpose of determining whether a
logical-end-of-tape (LEOT, encountered at the early-warning point) or a
physical-end-of-tape (PEOT) has been encountered; MT_EW and MT_EOT. With
MTANSI reset, writing is permitted from BOT to LEOT at which point both
MT_EW and MT_EOT and set. If MTANSI is now set, MT_EOT is reset (MT_EW
remains set) and writing is permitted until PEOT is encountered, at which
point MT_EOT is again set and remains set until the tape is repositioned
backward.
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mtio(7)mtio(7)IOCTL OPERATIONS
Different drivers and drives support different tape commands, in the form
of ioctl's. These operations and their structures are defined in the
file /usr/include/sys/mtio.h, which has fairly extensive comments. All
drivers support some common definitions of tape status via the MTIOCGET
ioctl; in particular, the bits defined for the mt_dposn field of the
mtget structure are the same for all the IRIX devices. other fields are
driver specific, and the appropriate header files should be consulted for
the meaning of these fields.
Those ioctls that are not supported for a particular drive or driver
normally return EINVAL. For example, the MTAFILE ioctl returns EINVAL if
the tape drive is not a device that supports overwrite (currently only
9-track and DAT), since QIC and 8mm drives only allow appending at EOD
and, for 8mm, from the BOT side of a FM.
NOTES
When a tape device is opened, the tape is rewound if there has been a
media change, or the drive has gone offline, or there has been a bus
reset (normally only after a reboot or powerup); otherwise the tape
remains at the same position as at the previous close. See the specific
driver manual pages for more details.
EXAMPLE
The following code fragment opens the default no rewind tape device,
positions it to the 2nd filemark, and then rewinds it. This is typical
of the use of most of the op codes for the MTIOCTOP and ABI_MTIOCTOP
ioctls; the latter is for use by programs compliant with the MIPS ABI
and, other than the name, functions identically to the former.
#include <sys/types.h>
#include <sys/mtio.h>
main()
{
struct mtop op;
int fd;
if((fd = open("/dev/nrtape", 0)) == -1)
perror("can't open /dev/tape");
op.mt_op = MTFSF;
op.mt_count = 2; /* number of fmk's to skip */
if(ioctl(fd, MTIOCTOP, &op) == -1)
perror("ioctl to skip 2 FMs fails");
op.mt_op = MTREW; /* mt_count field is ignored for MTREW */
if(ioctl(fd, MTIOCTOP, &op) == -1)
perror("ioctl to rewind fails");
}
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mtio(7)mtio(7)FILES
/dev/tape
/dev/nrtape
/dev/tapens
/dev/nrtapens
/dev/rmt/*
SEE ALSOMAKEDEV(1M), bru(1), cpio(1), mt(1), tar(1), dataudio(3), rmtops(3),
datframe(4), tps(7M).
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