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mtio(4)								       mtio(4)

Name
       mtio - 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.

Files
See Also
       lseek(2),  mu(4),  scsi(4),  stc(4),  tms(4),  ts(4),   tu(4),	tz(4),
       MAKEDEV(8)

								       mtio(4)
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