mtio.4

来自「<B>Digital的Unix操作系统VAX 4.2源码</B>」· 4 代码 · 共 271 行

.\" SCCSID: @(#)mtio.4	6.5	8/30/89
.TH mtio 4
.SH Name
mtio \- magnetic tape interface
.SH Description
.NXR "mtio keyword"
.NXR "magnetic tape interface"
.NXC "magnetic tape interface" "\fISee also interfaces for specific devices\fP
The 
.PN /dev 
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 different buses depending on the
bus/formatter/controller. On the BI, the TMSCP controllers,
.MS tms 4 ,
are available. 
On the DSSI bus the TMSCP controller
.MS tms 4
is available.
On the Q-bus the TMSCP controllers,
.MS tms 4 ,
and the TSV05 controller, 
.MS ts 4 ,
are available. On the MASSBUS, there are the TM03, 
.MS tu 4 ,
and TM78 formatters,
.MS mu 4 .
On the UNIBUS, TS11 formatters, the TSU05 controller, 
.MS ts 4 ,
and the TMSCP controllers, 
.MS tms 4 ,
are available.
On VAXstation 2000s and MicroVAX 2000s, the TZK50 cartridge tape subsystem, 
.MS stc 4 ,
is available.
On the SCSI bus, the SCSI tapes (see 
.MS tz 4 )
are available.
The following description applies to any mass
storage tape drive.
.PP
For both the ``rewind'' and \*Qnorewind\*U 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
.I 0
to
.I 31
for a total of
.I 32
tape drives.
.PP
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''.
.PP
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
.MS
tz 4
for further details.
.PP
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''.
.PP
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 operations 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.
.PP
The 
.B mtop 
data structure defined in 
.PN <sys/mtio.h> 
is passed as a
parameter to the MTIOCTOP ioctl.  The 
.B mtop 
structure is:
.EX 5
struct mtop {
	short   mt_op;
	daddr_t mt_count;
}
.EE
The mt_op field is used to specify the specific tape command to
be performed.  The mt_count field is used to specify the number of
times the command is to be performed (where applicable).
.PP
The following are tape operations supported in the MTIOCTOP ioctl.
These commands are specified in the mt_op field of the mtop structure.
.RS 5
.TP 10
MTWEOF
Writes an end-of-file to the tape.  Physically, an end of file consists of
a tape mark.  
.TP
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.
.TP
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 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 operations.  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.
.TP
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 indicates that there were not as many records remaining in
the file as specified by the mt_count parameter.
.TP
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 indicates that there were not as many records between the
present position and the beginning of the file as specified in the 
mt_count parameter.
.TP 
MTREW
Rewinds the tape.  This command repositions to the beginning of the tape.
.TP
MTOFFL
Rewinds and unloads the tape.
.TP
MTNOP
Does not perform any tape operation.  Always returns success from
a tape file.
.TP
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 operations 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.
.TP
MTNOCACHE
Disables use of the controller's write-back cache.  This mode of 
operation can result in performance degradation over cached mode.
.TP
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.
.TP
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
completion 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 
.I errno 
set to ENXIO means that the drive does not support the
flush command.  Failure with 
.I 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.
.TP
MTRETEN
Retensions the tape.  Retension means moving the tape one complete
pass between EOT and BOT.
.RE
.PP
The global variable 
.I errno 
is set to ENXIO if the command specified in
mt_op is not recognized or not supported by the respective tape driver.
.PP
Each
.PN read
or
.PN write
system call reads or writes the next record on the tape. In the case of
.PN write ,
the record has the same length as the buffer given.
During a 
.PN read , 
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 
.MS nbuf 4 ),
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 
.MS nbuf 4 ),
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 situation,
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
.I errno 
value is set.
.PP
Each
.PN read
or
.PN 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
.PN read
or
.PN write
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 
.PN lseek 
system call to reset the file offset to zero, for example, lseek(fd,0,0)
.SH Restrictions
For SCSI tapes on VAX systems, the maximum tape record
length is limited to 16K bytes (K = 1024).
.PP
For SCSI tapes on both VAX and RISC systems, the MTCACHE, MTNOCACHE, and 
MTFLUSH ioctls are not supported.
.PP
The MTRETEN ioctl is only supported by the SCSI QIC tape drive.
.SH Files
.PN /dev/rmt???
.br
.PN /dev/nrmt???
.SH See Also
lseek(2), mu(4), scsi(4), stc(4), tms(4), ts(4), tu(4), tz(4), MAKEDEV(8)