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ST(4)				    Linux Programmer's Manual				    ST(4)

       st - SCSI tape device

       #include <sys/mtio.h>

       int ioctl(int fd, int request [, (void *)arg3]);
       int ioctl(int fd, MTIOCTOP, (struct mtop *)mt_cmd);
       int ioctl(int fd, MTIOCGET, (struct mtget *)mt_status);
       int ioctl(int fd, MTIOCPOS, (struct mtpos *)mt_pos);

       The  st	driver	provides the interface to a variety of SCSI tape devices.  Currently, the
       driver takes control of all detected devices of type "sequential-access."  The  st  driver
       uses major device number 9.

       Each device uses eight minor device numbers. The lower-most five bits in the minor numbers
       are assigned sequentially in the order of detection. The minor numbers can be grouped into
       two  sets of four numbers: the principal (auto-rewind) minor device numbers, n, and a "no-
       rewind" device numbers, (n+ 128).  Devices opened using the principal device  number  will
       be  sent  a  REWIND  command  when  they are closed.  Devices opened using the "no-rewind"
       device number will not.	(Note that using an auto-rewind device for positioning	the  tape
       with,  for instance, mt does not lead to the desired result: the tape is rewound after the
       mt command and the next command starts from the beginning of the tape).

       Within each group, four minor numbers are available to define devices with different char-
       acteristics  (block  size, compression, density, etc.) When the system starts up, only the
       first device is available. The other three are activated when the default  characteristics
       are defined (see below). (By changing compile-time constants, it is possible to change the
       balance between the maximum number of tape drives and the number of minor numbers for each
       drive.  The default allocation allows control of 32 tape drives.  For instance, it is pos-
       sible to control up to 64 tape drives with two minor numbers for different options.)

       Devices are typically created by:
	      mknod -m 666 /dev/st0 c 9 0
	      mknod -m 666 /dev/st0l c 9 32
	      mknod -m 666 /dev/st0m c 9 64
	      mknod -m 666 /dev/st0a c 9 96
	      mknod -m 666 /dev/nst0 c 9 128
	      mknod -m 666 /dev/nst0l c 9 160
	      mknod -m 666 /dev/nst0m c 9 192
	      mknod -m 666 /dev/nst0a c 9 224

       There is no corresponding block device.

       The driver uses an internal buffer that has to be large enough to hold at least	one  tape
       block.  In  kernels  before 2.1.121, the buffer is allocated as one contiguous block. This
       limits the block size to the largest contiguous block of memory the kernel  allocator  can
       provide.   The  limit  is currently 128 kB for the 32-bit architectures and 256 kB for the
       64-bit architectures. In newer kernels the driver allocates the buffer in several parts if
       necessary.  By  default,  the  maximum  number of parts is 16. This means that the maximum
       block size is very large (2 MB if allocation of 16 blocks of 128 kB succeeds).

       The driver's internal buffer size is determined by a compile-time constant  which  can  be
       overridden  with  a kernel startup option.  In addition to this, the driver tries to allo-
       cate a larger temporary buffer at run-time if necessary. However, run-time  allocation  of
       large  contiguous  blocks  of  memory may fail and it is advisable not to rely too much on
       dynamic buffer allocation with kernels older than 2.1.121 (this applies	also  to  demand-
       loading the driver with kerneld or kmod).

       The  driver  does  not  specifically  support  any tape drive brand or model. After system
       start-up the tape device options are defined by the drive firmware. For	example,  if  the
       drive  firmware	selects  fixed	block  mode,  the  tape device uses fixed block mode. The
       options can be changed with explicit ioctl() calls and remain in effect when the device is
       closed  and reopened.  Setting the options affects both the auto-rewind and the non-rewind

       Different options can be specified for the different devices within the subgroup of  four.
       The  options  take effect when the device is opened. For example, the system administrator
       can define one device that writes in fixed block mode with a certain block size,  and  one
       which writes in variable block mode (if the drive supports both modes).

       The  driver  supports  tape  partitions if they are supported by the drive. (Note that the
       tape partitions have nothing to do with disk partitions. A partitioned tape can be seen as
       several	logical  tapes	within	one  medium.) Partition support has to be enabled with an
       ioctl. The tape location is preserved within each partition across partition changes.  The
       partition  used	for  subsequent  tape operations is selected with an ioctl. The partition
       switch is executed together with the next tape operation in  order  to  avoid  unnecessary
       tape  movement.	The  maximum  number of partitions on a tape is defined by a compile-time
       constant (originally four). The driver contains an ioctl  that  can  format  a  tape  with
       either one or two partitions.

       Device  /dev/tape  is usually created as a hard or soft link to the default tape device on
       the system.

       The driver supports operation in both fixed block mode and variable block  mode	(if  sup-
       ported  by  the	drive). In fixed block mode the drive writes blocks of the specified size
       and the block size is not dependent on the byte counts of the write system calls. In vari-
       able  block  mode  one tape block is written for each write call and the byte count deter-
       mines the size of the corresponding tape block. Note that the blocks on the tape do  don't
       contain	any information about the writing mode: when reading, the only important thing is
       to use commands that accept the block sizes on the tape.

       In variable block mode the read byte count does not have to  match  the	tape  block  size
       exactly.  If  the byte count is larger than the next block on tape, the driver returns the
       data and the function returns the actual block size. If the block size is larger than  the
       byte  count,  the requested amount of data from the start of the block is returned and the
       rest of the block is discarded.

       In fixed block mode the read byte counts can be arbitrary if buffering is  enabled,  or	a
       multiple  of  the  tape	block size if buffering is disabled. Kernels before 2.1.121 allow
       writes with arbitrary byte count if buffering is  enabled.  In  all  other  cases  (kernel
       before  2.1.121	with  buffering  disabled or newer kernel) the write byte count must be a
       multiple of the tape block size.

       A filemark is automatically written to tape if the last tape operation before close was	a

       When  a	filemark  is  encountered while reading, the following happens. If there are data
       remaining in the buffer when the filemark is found, the buffered  data  is  returned.  The
       next  read returns zero bytes. The following read returns data from the next file. The end
       of recorded data is signaled by returning zero bytes for two consecutive read  calls.  The
       third read returns an error.

       The  driver  supports  three ioctl requests.  Requests not recognized by the st driver are
       passed to the SCSI driver.  The definitions below are from /usr/include/linux/mtio.h:

   MTIOCTOP - Perform a tape operation
       This request takes an argument of type (struct mtop *).	Not all drives support all opera-
       tions.  The driver returns an EIO error if the drive rejects an operation.

       /* Structure for MTIOCTOP - mag tape op command: */
       struct mtop {
	   short  mt_op;    /* operations defined below */
	   int	  mt_count; /* how many of them */

       Magnetic Tape operations for normal tape use:
       MTBSF	     Backward space over mt_count filemarks.
       MTBSFM	     Backward space over mt_count filemarks.  Reposition the tape to the EOT side
		     of the last filemark.
       MTBSR	     Backward space over mt_count records (tape blocks).
       MTBSS	     Backward space over mt_count setmarks.
       MTCOMPRESSION Enable compression of tape data within the drive if mt_count is non-zero and
		     disable  compression if mt_count is zero. This command uses the MODE page 15
		     supported by most DATs.
       MTEOM	     Go to the end of the recorded media (for appending files).
       MTERASE	     Erase tape.
       MTFSF	     Forward space over mt_count filemarks.
       MTFSFM	     Forward space over mt_count filemarks.  Reposition the tape to the BOT  side
		     of the last filemark.
       MTFSR	     Forward space over mt_count records (tape blocks).
       MTFSS	     Forward space over mt_count setmarks.
       MTLOAD	     Execute  the  SCSI  load  command.  A  special case is available for some HP
		     autoloaders. If mt_count is the constant MT_ST_HPLOADER_OFFSET plus  a  num-
		     ber, the number is sent to the drive to control the autoloader.
       MTLOCK	     Lock the tape drive door.
       MTMKPART      Format  the  tape	into  one  or two partitions. If mt_count is non-zero, it
		     gives the size of the first partition and the second partition contains  the
		     rest of the tape. If mt_count is zero, the tape is formatted into one parti-
		     tion.  This command is not allowed for a drive unless the partition  support
		     is enabled for the drive (see MT_ST_CAN_PARTITIONS below).
       MTNOP	     No op - flushes the driver's buffer as a side effect.  Should be used before
		     reading status with MTIOCGET.
       MTOFFL	     Rewind and put the drive off line.
       MTRESET	     Reset drive.
       MTRETEN	     Retension tape.
       MTREW	     Rewind.
       MTSEEK	     Seek to the  tape	block  number  specified  in  mt_count.   This	operation
		     requires either a SCSI-2 drive that supports the LOCATE command (device-spe-
		     cific address) or a  Tandberg-compatible  SCSI-1  drive  (Tandberg,  Archive
		     Viper,  Wangtek,  ... ).  The block number should be one that was previously
		     returned by MTIOCPOS if device-specific addresses are used.
       MTSETBLK      Set the drive's block length to the value specified in  mt_count.	 A  block
		     length of zero sets the drive to variable block size mode.
       MTSETDENSITY  Set  the  tape density to the code in mt_count.  The density codes supported
		     by a drive can be found from the drive documentation.
       MTSETPART     The active partition is switched to mt_count .  The partitions are  numbered
		     from zero. This command is not allowed for a drive unless the partition sup-
		     port is enabled for the drive (see MT_ST_CAN_PARTITIONS below).
       MTUNLOAD      Execute the SCSI unload command (does not eject the tape).
       MTUNLOCK      Unlock the tape drive door.
       MTWEOF	     Write mt_count filemarks.
       MTWSM	     Write mt_count setmarks.

       Magnetic Tape operations for setting of device options (by the superuser):
	       Set various drive and driver options according to bits encoded in mt_count.  These
	       consist of the drive's buffering mode, 13 Boolean driver options, the buffer write
	       threshold, defaults for the block size and density, and timeouts (only in  kernels
	       >= 2.1).  A single operation can affect only one item in the list above (the Bool-
	       eans counted as one item.)

	       A value having zeros in the high-order 4 bits will be  used  to	set  the  drive's
	       buffering mode.	The buffering modes are:

		   0   The  drive  will  not  report GOOD status on write commands until the data
		       blocks are actually written to the medium.
		   1   The drive may report GOOD status on write commands as soon as all the data
		       has been transferred to the drive's internal buffer.
		   2   The  drive may report GOOD status on write commands as soon as (a) all the
		       data has been transferred to the drive's  internal  buffer,  and  (b)  all
		       buffered  data  from different initiators has been successfully written to
		       the medium.

	       To control the write threshold the value in mt_count  must  include  the  constant
	       MT_ST_WRITE_THRESHOLD  logically  ORed with a block count in the low 28 bits.  The
	       block count refers to 1024-byte blocks, not the physical block size on  the  tape.
	       The  threshold  cannot  exceed the driver's internal buffer size (see DESCRIPTION,

	       To set and clear the Boolean options the value in mt_count must	include  one  the
	       BOOLEANS logically ORed with whatever combination  of  the  following  options  is
	       desired.  Using MT_ST_BOOLEANS the options can be set to the values defined in the
	       corresponding bits. With MT_ST_SETBOOLEANS the options can be selectively set  and
	       with MT_ST_DEFBOOLEANS selectively cleared.

	       The default options for a tape device are set with MT_ST_DEFBOOLEANS. A non-active
	       tape device (e.g., device with minor 32 or 160)	is  activated  when  the  default
	       options	for  it are defined the first time. An activated device inherits from the
	       device activated at start-up the options not set explicitly.

	       The Boolean options are:

	       MT_ST_BUFFER_WRITES  (Default: true)
		      Buffer all write operations in fixed block mode.	If this option	is  false
		      and  the	drive  uses a fixed block size, then all write operations must be
		      for a multiple of the block size.  This option must be set false	to  write
		      reliable multi-volume archives.
	       MT_ST_ASYNC_WRITES  (Default: true)
		      When this options is true write operations return immediately without wait-
		      ing for the data to be transferred to the drive if the data fits	into  the
		      driver's	buffer.   The write threshold determines how full the buffer must
		      be before a new SCSI write command is issued.  Any errors reported  by  the
		      drive will be held until the next operation.  This option must be set false
		      to write reliable multi-volume archives.
	       MT_ST_READ_AHEAD  (Default: true)
		      This option causes the driver to provide read buffering and  read-ahead  in
		      fixed block mode.  If this option is false and the drive uses a fixed block
		      size, then all read operations must be for a multiple of the block size.
	       MT_ST_TWO_FM  (Default: false)
		      This option modifies the driver behavior when a file is closed.  The normal
		      action  is  to  write  a single filemark.  If the option is true the driver
		      will write two filemarks and backspace over the second one.

		      Note: This option should not be set true for QIC tape drives since they are
		      unable  to  overwrite  a filemark.  These drives detect the end of recorded
		      data by testing for blank tape rather than two consecutive filemarks.  Most
		      other  current  drives  also  detect the end of recorded data and using two
		      filemarks is usually necessary only  when  interchanging	tapes  with  some
		      other systems.

	       MT_ST_DEBUGGING	(Default: false)
		      This  option turns on various debugging messages from the driver (effective
		      only if the driver was compiled with DEBUG defined non-zero).
	       MT_ST_FAST_EOM  (Default: false)
		      This option causes the MTEOM operation to be sent directly  to  the  drive,
		      potentially  speeding up the operation but causing the driver to lose track
		      of the current file number normally returned by the MTIOCGET  request.   If
		      MT_ST_FAST_EOM is false the driver will respond to an MTEOM request by for-
		      ward spacing over files.
	       MT_ST_AUTO_LOCK (Default: false)
		      When this option is true, the drive door	is  locked  when  the  device  is
		      opened and unlocked when it is closed.
	       MT_ST_DEF_WRITES (Default: false)
		      The  tape  options  (block  size,  mode, compression, etc.) may change when
		      changing from one device linked to a drive to another device linked to  the
		      same  drive  depending  on how the devices are defined. This option defines
		      when the changes are enforced by the driver using  SCSI-commands	and  when
		      the  drives  auto-detection capabilities are relied upon. If this option is
		      false, the driver sends the SCSI-commands immediately when  the  device  is
		      changed.	If  the  option  is  true, the SCSI-commands are not sent until a
		      write is requested. In this case the drive firmware is  allowed  to  detect
		      the tape structure when reading and the SCSI-commands are used only to make
		      sure that a tape is written according to the correct specification.
	       MT_ST_CAN_BSR (Default: false)
		      When read-ahead is used, the tape must sometimes be spaced backward to  the
		      correct  position  when  the device is closed and the SCSI command to space
		      backwards over records is used for this purpose. Some  older  drives  can't
		      process  this  command reliably and this option can be used to instruct the
		      driver not to use the command. The end result is that, with read-ahead  and
		      fixed  block  mode,  the tape may not be correctly positioned within a file
		      when the device is closed.
	       MT_ST_NO_BLKLIMS (Default: false)
		      Some drives don't accept the READ BLOCK LIMITS SCSI  command.  If  this  is
		      used,  the driver does not use the command. The drawback is that the driver
		      can't check before sending commands if the selected block size  is  accept-
		      able to the drive.
	       MT_ST_CAN_PARTITIONS (Default: false)
		      This  option  enables  support  for  several  partitions within a tape. The
		      option applies to all devices linked to a drive.
	       MT_ST_SCSI2LOGICAL (Default: false)
		      This option instructs the driver to use the logical block addresses defined
		      in  the  SCSI-2 standard when performing the seek and tell operations (both
		      with MTSEEK and MTIOCPOS commands and when changing tape partition). Other-
		      wise the device-specific addresses are used.  It is highly advisable to set
		      this option if the drive supports the logical addresses because they  count
		      also  filemarks.	There are some drives that only support the logical block
	       MT_ST_SYSV (Default: false)
		      When this option is enabled, the tape devices use  the  SystemV  semantics.
		      Otherwise the BSD semantics are used. The most important difference between
		      the semantics is what happens when a device used for reading is closed:  in
		      SYSV  semantics  the  tape is spaced forward past the next filemark if this
		      has not happened while using the device. In BSD semantics the tape position
		      is not changed.
		      struct mtop mt_cmd;
		      mt_cmd.mt_op = MTSETDRVBUFFER;
		      mt_cmd.mt_count = MT_ST_BOOLEANS |
		      ioctl(fd, MTIOCTOP, &mt_cmd);

	       The  default  block  size  for  a device can be set with MT_ST_DEF_BLKSIZE and the
	       default density code can be set with MT_ST_DEFDENSITY. The values for the  parame-
	       ters are ORed with the operation code.

	       With  kernels  2.1.x  and later, the timeout values can be set with the subcommand
	       MT_ST_SET_TIMEOUT or'ed with the timeout in seconds.  The long timeout  (used  for
	       rewinds	and  other  commands  that  may  take  a  long	time)  can  be	set  with
	       MT_ST_SET_LONG_TIMEOUT. The kernel defaults are very long to make sure that a suc-
	       cessful	command  is  not timed out with any drive. Because of this the driver may
	       seem stuck even if it is only waiting for the timeout. These commands can be  used
	       to set more practical values for a specific drive. The timeouts set for one device
	       apply for all devices linked to the same drive.

   MTIOCGET - Get status
       This request takes an argument of type (struct mtget *).

       /* structure for MTIOCGET - mag tape get status command */
       struct mtget {
	   long   mt_type;
	   long   mt_resid;
	   /* the following registers are device dependent */
	   long   mt_dsreg;
	   long   mt_gstat;
	   long   mt_erreg;
	   /* The next two fields are not always used */
	   daddr_t	    mt_fileno;
	   daddr_t	    mt_blkno;

       mt_type	  The header file defines many values for mt_type, but the current driver reports
		  only the generic types MT_ISSCSI1 (Generic SCSI-1 tape) and MT_ISSCSI2 (Generic
		  SCSI-2 tape).
       mt_resid   contains the current tape partition number.
       mt_dsreg   reports the drive's current settings for block size (in the low  24  bits)  and
		  density (in the high 8 bits).  These fields are defined by MT_ST_BLKSIZE_SHIFT,
       mt_gstat   reports generic (device  independent)  status  information.	The  header  file
		  defines macros for testing these status bits:
		  GMT_EOF(x): The tape is positioned just after a filemark (always false after an
		      MTSEEK operation).
		  GMT_BOT(x): The tape is positioned at the beginning of the first  file  (always
		      false after an MTSEEK operation).
		  GMT_EOT(x): A tape operation has reached the physical End Of Tape.
		  GMT_SM(x): The tape is currently positioned at a setmark (always false after an
		      MTSEEK operation).
		  GMT_EOD(x): The tape is positioned at the end of recorded data.
		  GMT_WR_PROT(x): The drive is write-protected.  For some drives  this	can  also
		      mean that the drive does not support writing on the current medium type.
		  GMT_ONLINE(x):  The  last open() found the drive with a tape in place and ready
		      for operation.
		  GMT_D_6250(x), GMT_D_1600(x), GMT_D_800(x): This "generic"  status  information
		      reports the current density setting for 9-track 1/2" tape drives only.
		  GMT_DR_OPEN(x): The drive does not have a tape in place.
		  GMT_IM_REP_EN(x):  Immediate report mode. This bit is set if there are no guar-
		      antees that the data has been physically written to the tape when the write
		      call  returns. It is set zero only when the driver does not buffer data and
		      the drive is set not to buffer data.
       mt_erreg   The only field defined in mt_erreg is the recovered error count in the  low  16
		  bits (as defined by MT_ST_SOFTERR_SHIFT and MT_ST_SOFTERR_MASK).  Due to incon-
		  sistencies in the way drives report recovered errors, this count is  often  not
		  maintained  (most  drives  do not by default report soft errors but this can be
		  changed with a SCSI MODE SELECT command).
       mt_fileno  reports the current file number (zero-based).  This value is set to -1 when the
		  file number is unknown (e.g., after MTBSS or MTSEEK).
       mt_blkno   reports  the	block number (zero-based) within the current file.  This value is
		  set to -1 when the block number  is  unknown	(e.g.,	after  MTBSF,  MTBSS,  or

   MTIOCPOS - Get tape position
       This  request takes an argument of type (struct mtpos *) and reports the drive's notion of
       the current tape block number, which is not the same as	mt_blkno  returned  by	MTIOCGET.
       This drive must be a SCSI-2 drive that supports the READ POSITION command (device-specific
       address) or a Tandberg-compatible SCSI-1 drive (Tandberg, Archive Viper, Wangtek, ... ).

       /* structure for MTIOCPOS - mag tape get position command */
       struct	  mtpos {
	   long   mt_blkno; /* current block number */

       EIO	     The requested operation could not be completed.

       ENOSPC	     A write operation could not be completed because the  tape  reached  end-of-

       EACCES	     An  attempt  was made to write or erase a write-protected tape.  (This error
		     is not detected during open().)

       EFAULT	     The command parameters point to memory not belonging to the calling process.

       ENXIO	     During opening, the tape device does not exist.

       EBUSY	     The device is already in use or the driver was unable to allocate a buffer.

       EOVERFLOW     An attempt was made to read or write a variable-length block that is  larger
		     than the driver's internal buffer.

       EINVAL	     An ioctl() had an illegal argument, or a requested block size was illegal.

       ENOSYS	     Unknown ioctl().

       EROFS	     Open  is  attempted  with	O_WRONLY  or O_RDWR when the tape in the drive is

       /dev/st*  : the auto-rewind SCSI tape devices
       /dev/nst* : the non-rewind SCSI tape devices

       The driver has been written by Kai Makisara (Kai.Makisara@metla.fi) starting from a driver
       written by Dwayne Forsyth. Several other people have also contributed to the driver.


       The  file  README.st  in the kernel sources contains the most recent information about the
       driver and its configuration possibilities.

       1. When exchanging data between systems, both systems have to agree on the  physical  tape
       block  size. The parameters of a drive after startup are often not the ones most operating
       systems use with these devices. Most systems use drives in  variable  block  mode  if  the
       drive  supports that mode. This applies to most modern drives, including DATs, 8mm helical
       scan drives, DLTs, etc. It may be advisable use these drives in variable block  mode  also
       in  Linux  (i.e., use MTSETBLK or MTSETDEFBLK at system startup to set the mode), at least
       when exchanging data with foreign system. The drawback of this is that a fairly large tape
       block size has to be used to get acceptable data transfer rates on the SCSI bus.

       2.  Many  programs  (e.g.,  tar)  allow the user to specify the blocking factor on command
       line. Note that this determines the physical block size on tape	only  in  variable  block

       3.  In order to use SCSI tape drives, the basic SCSI driver, a SCSI-adapter driver and the
       SCSI tape driver must be either configured into the kernel or loaded as	modules.  If  the
       SCSI-tape driver is not present, the drive is recognized but the tape support described in
       this page is not available.

       4. The driver writes error messages to the console/log. The SENSE codes written into  some
       messages are automatically translated to text if verbose SCSI messages are enabled in ker-
       nel configuration.

       Copyright (C) 1995 Robert K. Nichols.
       Copyright (C) 1999 Kai Makisara.

       Permission is granted to make and distribute verbatim copies of this manual  provided  the
       copyright  notice and this permission notice are preserved on all copies.  Additional per-
       missions are contained in the header of the source file.

Linux 2.0 - 2.2 			    1999-01-18					    ST(4)
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