#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 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.
       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
       nonrewind 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 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

       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 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

       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.

       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  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       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 drive's 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

                   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_DEFBOOLEANS  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_DEF-
               BOOLEANS.  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 opera-
                      tions return immediately without  waiting
                      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
                      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

                      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  test-
                      ing  for  blank tape rather than two con-
                      secutive 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,  poten-
                      tially  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  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, com-
                      pression, 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-com-
                      mands 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
                      Some older drives can't process this com-
                      mand 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 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 performing 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 supports 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.  Oth-
                      erwise 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 hap-
                      pened while using  the  device.   In  BSD
                      semantics   the   tape  position  is  not

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

               ues for the parameters are or'ed with the opera-
               tion 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  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 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
                  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

                  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

                  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  guaran-
                      tees  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

       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
                  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 sup-
       ports  the  READ   POSITION   command   (device-specific
       address)  or  a  Tandberg-compatible SCSI-1 drive (Tand-
       berg, 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-protected   tape.
                     (This error is not detected  during

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

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

       EINVAL        An  ioctl(2)  had  an invalid argu-
                     ment, or a requested block size was

       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 com-
                     pleted  because  the  tape  reached

       ENOSYS        Unknown ioctl(2).

       ENXIO         During  opening,  the  tape  device
                     does not exist.

       EOVERFLOW     An  attempt  was  made  to  read or

       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 heli-
           cal scan drives, DLTs, etc.  It may be advis-
           able  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

       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  con-
           sole/log.   The SENSE codes written into some
           messages are automatically translated to text
           if  verbose SCSI messages are enabled in ker-
           nel 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  ker-
           nel.  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.


       The  file  drivers/scsi/  or  Documenta-

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