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ST(4)			   Linux Programmers Manual			ST(4)

       st - SCSI tape device


       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 lowermost five bits
       in the minor numbers are assigned sequentially in the order  of	detec
       tion.   In  the 2.6 kernel, the bits above the eight lowermost bits are
       concatenated to the five lowermost bits to form the tape  number.   The
       minor numbers can be grouped into two sets of four numbers: the princi
       pal (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 com
       mand and the next command starts from the beginning of the tape).

       Within each group, four minor numbers are available to  define  devices
       with different characteristics (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 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 drivers internal buffer size is determined by a compile-time  con
       stant  which  can be overridden with a kernel startup option.  In addi
       tion to this, the driver tries to allocate a larger temporary buffer at
       run  time if necessary.	However, run-time allocation of large contigu
       ous 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 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(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 supported 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 deter
       mines the size of the corresponding tape block.	Note that  the	blocks
       on  the tape dont 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 dis
       abled.  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  drivers
       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.

       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  operations.  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

       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	avail
		     able  for	some  HP  autoloaders.	 If  mt_count  is  the
		     constant MT_ST_HPLOADER_OFFSET plus a number, 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 drivers 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 sup
		     ports the LOCATE command (device-specific 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-spe
		     cific addresses are used.

       MTSETBLK      Set the drives block length to the  value	specified  in
		     mt_count.	A block length of zero sets the drive to vari
		     able block size mode.

       MTSETDENSITY  Set the tape density to the code in mt_count.   The  den
		     sity  codes  supported  by  a drive can be found from the
		     drive documentation.

       MTSETPART     The active partition is switched to mt_count.  The parti
		     tions  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

       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  supe

	       Set  various drive and driver options according to bits encoded
	       in mt_count.  These consist of the drives  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 drives 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

		   1   The  drive  may report GOOD status on write commands as
		       soon as all  the  data  has  been  transferred  to  the
		       drives internal buffer.

		   2   The  drive  may report GOOD status on write commands as
		       soon as (a) all the data has been  transferred  to  the
		       drives 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 drivers 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_DEFBOOLEANS 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_DEF
	       BOOLEANS.  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  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 drivers 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

	       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

	       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 file
		      mark.  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	neces
		      sary  only when interchanging tapes with some other sys

	       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 oper
		      ation but causing the driver to lose track of  the  cur
		      rent  file  number  normally  returned  by  the MTIOCGET
		      request.	If MT_ST_FAST_EOM is  false  the  driver  will
		      respond  to  an  MTEOM  request  by forward spacing over

	       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 depend
		      ing 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

	       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
		      cant 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 dont accept the READ BLOCK LIMITS SCSI com
		      mand.  If this is used, the driver does not use the com
		      mand.   The  drawback  is  that  the  driver cant check
		      before sending commands if the selected  block  size  is
		      acceptable 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 per
		      forming  the  seek and tell operations (both with MTSEEK
		      and MTIOCPOS commands and when changing tape partition).
		      Otherwise the device-specific addresses are used.  It is
		      highly advisable to set this option if  the  drive  sup
		      ports  the  logical  addresses  because  they count also
		      filemarks.  There are some drives that only support  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 seman
		      tics 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., dont 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 |
		   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 parameters 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 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_TIME
	       OUT.  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
	       TapeAlert 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 pattern is non-zero, 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 drives 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_gstat   reports  generic  (device  independent)  status information.
		  The header file defines  macros  for	testing  these	status

		  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

		  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

		  GMT_WR_PROT(x): The  drive  is  write-protected.   For  some
		      drives  this  can also mean that the drive does not sup
		      port 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 " 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 guarantees that the data  has  been	physi
		      cally  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 inconsistencies 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
       drives 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 */

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

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

       EFAULT	     The command parameters point to memory not belong
		     ing 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

       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  drivers
		     internal buffer.

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

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

       /dev/nst*   the non-rewind SCSI tape devices

       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  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

       2.  Many programs (e.g., tar(1)) allow the user to  specify  the
	   blocking  factor on the command line.  Note that this deter
	   mines 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
	   kernel configuration.

       5.  The drivers 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 solu
	   tion 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.


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

       This  page  is  part  of  release  3.05	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-

Linux				  2007-12-16				 ST(4)

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