174 lines
5.8 KiB
Groff
174 lines
5.8 KiB
Groff
.\" $NetBSD: mtio.4,v 1.10 1999/09/12 18:43:43 kleink Exp $
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.\"
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.\" Copyright (c) 1983, 1991, 1993
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.\" The Regents of the University of California. All rights reserved.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\" 3. All advertising materials mentioning features or use of this software
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.\" must display the following acknowledgement:
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.\" This product includes software developed by the University of
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.\" California, Berkeley and its contributors.
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.\" 4. Neither the name of the University nor the names of its contributors
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.\" may be used to endorse or promote products derived from this software
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.\" without specific prior written permission.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" from: @(#)mtio.4 8.1 (Berkeley) 6/5/93
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.\"
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.Dd January 14, 1999
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.Dt MTIO 4
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.Os
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.Sh NAME
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.Nm mtio
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.Nd generic magnetic tape I/O interface
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.Sh SYNOPSIS
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.Cd "#include <sys/mtio.h>"
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.Sh DESCRIPTION
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Magnetic tape has been the computer system backup and data transfer
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medium of choice for decades, because it has historically been
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cheaper in cost per bit stored, and the formats have been designed
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for portability and storage.
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However, tape drives have generally been the slowest mass
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storage devices attached to any computer system.
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.Pp
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Magnetic tape comes in a wide variety of formats, from classic 9-track,
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through various Quarter Inch Cartridge
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.Pq Tn QIC
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variants, to more modern systems using 8mm video tape, and
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Digital Audio Tape
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.Pq Tn DAT .
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There have also been a variety of proprietary tape systems, including
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.Tn DECtape ,
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and
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.Tn "IBM 3480" .
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.Ss UNIX TAPE I/O
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Regardless of the specific characteristics of the particular tape
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transport mechanism (tape drive),
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.Ux
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tape I/O has two interfaces:
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.Qq block
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and
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.Qq raw .
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I/O through the block interface of a tape device is similar to I/O
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through the block special device for a disk driver: the individual
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.Xr read 2
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and
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.Xr write 2
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calls can be done in any amount of bytes, but all data is buffered
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through the system buffer cache, and I/O to the device is done in
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1024 byte sized blocks.
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This limitation is sufficiently restrictive that the block interface
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to tape devices is rarely used.
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.Pp
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The
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.Qq raw
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interface differs in that all I/O can be done in arbitrary sized blocks,
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within the limitations for the specific device and device driver,
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and all I/O is synchronous.
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This is the most flexible interface, but since there is very little
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that is handled automatically by the kernel, user programs must
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implement specific magnetic tape handling routines, which puts the onus
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of correctness on the application programmer.
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.Ss DEVICE NAME CONVENTIONS
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Each magnetic tape subsystem has a couple of special devices
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associated with it.
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.Pp
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The block device is usually named for the driver, e.g.
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.Pa /dev/st0
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for unit zero of a
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.Xr st 4
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.Tn SCSI
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tape drive.
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.Pp
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The raw device name is the block device name with an
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.Qq r
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prepended, e.g.
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.Pa /dev/rst0 .
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.Pp
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By default, the tape driver will rewind the tape drive when the
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device is closed.
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To make it possible for multiple program invocations to
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sequentially write multiple files on the same tape, a
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.Qq no rewind on close
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device is provided, denoted by the letter
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.Qq n
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prepended to the name of the device, e.g.
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.Pa /dev/nst0 ,
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.Pa /dev/nrst0 .
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.Pp
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The
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.Xr mt 1
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command can be used to explicitly rewind, or otherwise position a
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tape at a particular point with the no-rewind device.
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.Ss FILE MARK HANDLING
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Two end-of-file (EOF) markers mark the end of a tape (EOT), and
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one end-of-file marker marks the end of a tape file.
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.Pp
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By default, the tape driver will write two End Of File (EOF) marks
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and rewind the tape when the device is closed after the last write.
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.Pp
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If the tape is not to be rewound it is positioned with the
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head in between the two tape marks, where the next write
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will over write the second end-of-file marker.
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.Pp
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All of the magnetic tape devices may be manipulated with the
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.Xr mt 1
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command.
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.Pp
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A number of
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.Xr ioctl 2
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operations are available on raw magnetic tape.
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Please see
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.Aq Pa sys/mtio.h
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for their definitions.
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.\" The following definitions are from
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.\" .Aq Pa sys/mtio.h :
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.\" .Bd -literal
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.\" there was a copy of sys/mtio.h here. silly.
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.\" .Ed
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.Pp
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The manual pages for specific tape device drivers should list their
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particular capabilities and limitations.
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.Sh SEE ALSO
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.Xr dd 1 ,
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.Xr mt 1 ,
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.Xr tar 1 ,
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.Xr pax 1 ,
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.Xr ht 4 ,
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.Xr st 4 ,
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.Xr tm 4 ,
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.Xr ts 4 ,
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.Xr mt 4 ,
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.Xr ut 4
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.Sh HISTORY
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The
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.Nm
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manual appeared in
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.Bx 4.2 .
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.Sh BUGS
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The status should be returned in a device independent format.
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.Pp
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If and when
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.Nx
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is updated to deal with non-512 byte per sector disk media through the
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system buffer cache, perhaps a more sane tape interface can be
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implemented.
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