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

NAME
       st - SCSI tape device

SYNOPSIS
       #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);

DESCRIPTION
       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 lowermost five bits in the minor numbers
       are assigned sequentially in the order of detection.  In the 2.6 kernel,  the  bits  above
       the eight lowermost bits are concatenated to the five lowermost bits to form the tape num-
       ber.  The minor numbers can be grouped into two sets of four numbers: the principal (auto-
       rewind)	minor  device numbers, n, and the "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
       possible 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 32-bit architectures and 256 kB for 64-bit
       architectures.  In newer kernels the driver allocates the buffer in several parts if  nec-
       essary.	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(2) calls and remain in effect when  the  device
       is  closed  and	reopened.   Setting the options affects both the auto-rewind and the non-
       rewind device.

       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(2).  The tape location is preserved within each partition across partition  changes.
       The  partition used for subsequent tape operations is selected with an ioctl(2).  The par-
       tition switch is executed together with the next tape operation in order to avoid unneces-
       sary  tape  movement.  The maximum number of partitions on a tape is defined by a compile-
       time constant (originally four).  The driver contains an ioctl(2) 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.

       Starting from kernel 2.6.2, the driver exports in the sysfs directory /sys/class/scsi_tape
       the attached devices and some parameters assigned to the devices.

   Data transfer
       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
       variable-block mode one tape block is written for each  write  call  and  the  byte  count
       determines  the	size  of  the corresponding tape block.  Note that the blocks on the tape
       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.

       In  the	2.6  kernel, the driver tries to use direct transfers between the user buffer and
       the device.  If this is not possible, the driver's internal buffer is used.   The  reasons
       for  not  using direct transfers include improper alignment of the user buffer (default is
       512 bytes but this can be changed by the HBA driver), one of more pages of the user buffer
       not reachable by the SCSI adapter, etc.

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

       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.

   Ioctls
       The driver supports three ioctl(2) 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 nonzero 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.  With 2.6 kernel, short erase (mark tape empty) is performed if
		     the argument is zero.  Otherwise long erase (erase all) is done.

       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 nonzero, 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 par-
		     tition.  This command is not allowed for a drive unless the  partition  sup-
		     port 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	     Re-tension 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
		     support 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):

       MTSETDRVBUFFER
	       Set various drive and driver options according to bits encoded in mt_count.  These
	       consist of the drive's buffering mode, a set of Boolean driver options, the buffer
	       write threshold, defaults for the block size and density, and  timeouts	(only  in
	       kernels	2.1  and later).  A single operation can affect only one item in the list
	       above (the Booleans 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  bitwise  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,
	       above).

	       To set and clear the Boolean options the value in mt_count must include one of the
	       constants  MT_ST_BOOLEANS,  MT_ST_SETBOOLEANS,  MT_ST_CLEARBOOLEANS, or MT_ST_DEF-
	       BOOLEANS bitwise 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 nonactive
	       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 multivolume archives.

	       MT_ST_ASYNC_WRITES (Default: true)
		      When this option 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 multivolume 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 nonzero).

	       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
		      backward 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.  With 2.6 kernel, the default is true for drives
		      supporting SCSI-3.

	       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).  Oth-
		      erwise 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 support only the  logical
		      block addresses.

	       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 System V 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.

	       MT_NO_WAIT (Default: false)
		      Enables immediate mode (i.e., don't wait for the	command  to  finish)  for
		      some commands (e.g., rewind).

	       An example:

		   struct mtop mt_cmd;
		   mt_cmd.mt_op = MTSETDRVBUFFER;
		   mt_cmd.mt_count = MT_ST_BOOLEANS |
			   MT_ST_BUFFER_WRITES | MT_ST_ASYNC_WRITES;
		   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 or'ed with the operation code.

	       With  kernels  2.1.x  and later, the timeout values can be set with the subcommand
	       MT_ST_SET_TIMEOUT ORed 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
	       successful  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.

	       Starting from kernels 2.4.19 and 2.5.43, the driver supports a  status  bit  which
	       indicates  whether  the	drive requests cleaning.  The method used by the drive to
	       return cleaning information is set using the  MT_ST_SEL_CLN  subcommand.   If  the
	       value  is zero, the cleaning bit is always zero.  If the value is one, the TapeAl-
	       ert data defined in the SCSI-3 standard is used	(not  yet  implemented).   Values
	       2-17  are  reserved.   If  the lowest eight bits are >= 18, bits from the extended
	       sense data are used.  The bits 9-16 specify a mask to select the bits to  look  at
	       and  the  bits  17-23  specify the bit pattern to look for.  If the bit pattern is
	       zero, one or more bits under the mask indicate the cleaning request.  If the  pat-
	       tern is nonzero, the pattern must match the masked sense data byte.

   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_ST_BLKSIZE_MASK, MT_ST_DENSITY_SHIFT, and MT_ST_DENSITY_MASK.

       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(2) 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.

		  GMT_CLN(x):  The  drive  has	requested cleaning.  Implemented in kernels since
		      2.4.19 and 2.5.43.

       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
		  MTSEEK).

   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 */
	   };

RETURN VALUE
       EACCES	     An attempt was made to write or erase a write-protected tape.   (This
		     error is not detected during open(2).)

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

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

       EINVAL	     An  ioctl(2)  had	an invalid argument, or a requested block size was
		     invalid.

       EIO	     The requested operation could not be completed.

       ENOMEM	     The byte count in read(2) is smaller than the next physical block	on
		     the  tape.   (Before 2.2.18 and 2.4.0-test6 the extra bytes have been
		     silently ignored.)

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

       ENOSYS	     Unknown ioctl(2).

       ENXIO	     During opening, the tape device does not exist.

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

       EROFS	     Open is attempted with O_WRONLY or O_RDWR when the tape in the  drive
		     is write-protected.

FILES
       /dev/st*    the auto-rewind SCSI tape devices

       /dev/nst*   the nonrewind SCSI tape devices

NOTES
       1.  When  exchanging data between systems, both systems have to agree on the physi-
	   cal 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 mod-
	   ern	drives,  including  DATs,  8mm	helical scan drives, DLTs, etc.  It may be
	   advisable to 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 a 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(1)) allow the user to specify the blocking	factor	on
	   the	command  line.	 Note that this determines the physical block size on tape
	   only in variable-block mode.

       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  mes-
	   sages are enabled in kernel configuration.

       5.  The driver's internal buffering allows good throughput in fixed-block mode also
	   with small read(2) and write(2) byte counts.  With direct transfers this is not
	   possible  and may cause a surprise when moving to the 2.6 kernel.  The solution
	   is to tell the software to use larger transfers (often telling it to use larger
	   blocks).  If this is not possible, direct transfers can be disabled.

SEE ALSO
       mt(1)

       The file drivers/scsi/README.st or Documentation/scsi/st.txt (kernel >= 2.6) in the
       Linux kernel source tree contains the most recent information about the driver  and
       its configuration possibilities

COLOPHON
       This page is part of release 3.55 of the Linux man-pages project.  A description of
       the  project,  and  information	 about	 reporting   bugs,   can   be	found	at
       http://www.kernel.org/doc/man-pages/.

Linux					    2010-09-04					    ST(4)
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