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       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 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  number.
       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 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 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  allocate  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
       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
       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.

       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:

   MTIOCTOPPerform 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.

       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 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 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
                     partition.   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
                     supports 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-
                     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

       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

               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

                   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_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

               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
                      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 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

                      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  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 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 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.
                      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

               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 parameters  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 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.

   MTIOCGETGet 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_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

                  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
                      ½" 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
                      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
                  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).

   MTIOCPOSGet 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 */


       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
                     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

       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.


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

       /dev/nst*   the nonrewind 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 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
           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  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.



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


       This  page  is  part of release 3.35 of the Linux
       man-pages project.  A description of the project,
       and  information  about  reporting  bugs,  can be
       found at