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NAME

       st - SCSI tape device

SYNOPSIS

       #include <sys/mtio.h>

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

DESCRIPTION

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

       Each  device  uses eight minor device numbers.  The lowermost five bits
       in the  minor  numbers  are  assigned  sequentially  in  the  order  of
       detection.   In the 2.6 kernel, the bits above the eight lowermost bits
       are concatenated to the five lowermost bits to form  the  tape  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
       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
       determines  the  size  of  the corresponding tape block.  Note that the
       blocks on the tape don’t contain  any  information  about  the  writing
       mode:  when  reading,  the only important thing is to use commands that
       accept the block sizes on the tape.

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

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

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

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

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

   Ioctls
       The  driver  supports three ioctl(2) requests.  Requests not recognized
       by the st driver are passed to the SCSI driver.  The definitions  below
       are from /usr/include/linux/mtio.h:

   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 non-zero and disable compression if mt_count
                     is zero.  This command uses the MODE page 15 supported by
                     most DATs.

       MTEOM         Go  to  the  end  of  the  recorded  media (for appending
                     files).

       MTERASE       Erase tape.  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 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
                     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
                     tape).

       MTUNLOCK      Unlock the tape drive door.

       MTWEOF        Write mt_count filemarks.

       MTWSM         Write mt_count setmarks.

       Magnetic  Tape  operations  for  setting  of  device  options  (by  the
       superuser):

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

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

                   0   The drive will not report GOOD status on write commands
                       until  the  data  blocks  are  actually  written to the
                       medium.

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

                   2   The drive may report GOOD status on write  commands  as
                       soon  as  (a)  all the data has been transferred to the
                       drive’s internal buffer, and (b) all buffered data from
                       different  initiators  has been successfully written to
                       the medium.

               To control the write  threshold  the  value  in  mt_count  must
               include  the constant MT_ST_WRITE_THRESHOLD logically ORed with
               a block count in the low 28 bits.  The block  count  refers  to
               1024-byte blocks, not the physical block size on the tape.  The
               threshold cannot exceed the driver’s internal buffer size  (see
               DESCRIPTION, 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 or’ed with
               whatever combination  of  the  following  options  is  desired.
               Using  MT_ST_BOOLEANS  the  options  can  be  set to the values
               defined in the corresponding bits.  With MT_ST_SETBOOLEANS  the
               options  can  be  selectively  set  and  with MT_ST_DEFBOOLEANS
               selectively cleared.

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

               The Boolean options are:

               MT_ST_BUFFER_WRITES (Default: true)
                      Buffer  all  write  operations  in fixed-block mode.  If
                      this option is false and the drive uses  a  fixed  block
                      size,  then  all write operations must be for a multiple
                      of the block size.  This option must  be  set  false  to
                      write        reliable       multi-volume       archives.
                      MT_ST_ASYNC_WRITES (Default: true) When this  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 multi-
                      volume archives.

               MT_ST_READ_AHEAD (Default: true)
                      This option causes the driver to provide read  buffering
                      and  read-ahead  in fixed-block mode.  If this option is
                      false and the drive uses a fixed block  size,  then  all
                      read  operations  must  be  for  a multiple of the block
                      size.

               MT_ST_TWO_FM (Default: false)
                      This option modifies the driver behavior when a file  is
                      closed.    The  normal  action  is  to  write  a  single
                      filemark.  If the option is true the driver  will  write
                      two filemarks and backspace over the second one.

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

               MT_ST_DEBUGGING (Default: false)
                      This option turns on various debugging messages from the
                      driver  (effective  only if the driver was compiled with
                      DEBUG defined non-zero).

               MT_ST_FAST_EOM (Default: false)
                      This option  causes  the  MTEOM  operation  to  be  sent
                      directly  to  the  drive,  potentially  speeding  up the
                      operation but causing the driver to lose  track  of  the
                      current  file  number  normally returned by the MTIOCGET
                      request.  If MT_ST_FAST_EOM is  false  the  driver  will
                      respond  to  an  MTEOM  request  by forward spacing over
                      files.

               MT_ST_AUTO_LOCK (Default: false)
                      When this option is true, the drive door is locked  when
                      the device is opened and unlocked when it is closed.

               MT_ST_DEF_WRITES (Default: false)
                      The  tape  options (block size, mode, compression, etc.)
                      may change when changing from one  device  linked  to  a
                      drive  to  another  device  linked  to  the  same  drive
                      depending on how the devices are defined.   This  option
                      defines  when  the  changes  are  enforced by the driver
                      using SCSI-commands and when the  drives  auto-detection
                      capabilities  are relied upon.  If this option is false,
                      the driver sends the SCSI-commands immediately when  the
                      device  is  changed.   If  the option is true, the SCSI-
                      commands are not sent until a write  is  requested.   In
                      this  case  the  drive firmware is allowed to detect the
                      tape structure when reading and  the  SCSI-commands  are
                      used  only to make sure that a tape is written according
                      to the correct specification.

               MT_ST_CAN_BSR (Default: false)
                      When read-ahead is used,  the  tape  must  sometimes  be
                      spaced  backward to the correct position when the device
                      is closed and the SCSI command to space  backwards  over
                      records  is  used  for  this purpose.  Some older drives
                      can’t process this command reliably and this option  can
                      be  used  to instruct the driver not to use the command.
                      The end result is that, with read-ahead and  fixed-block
                      mode,  the tape may not be correctly positioned within a
                      file when the device is closed.  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 changed.

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

               An example:

                   struct mtop mt_cmd;
                   mt_cmd.mt_op = MTSETDRVBUFFER;
                   mt_cmd.mt_count = MT_ST_BOOLEANS |
                           MT_ST_BUFFER_WRITES | MT_ST_ASYNC_WRITES;
                   ioctl(fd, MTIOCTOP, mt_cmd);

               The   default   block  size  for  a  device  can  be  set  with
               MT_ST_DEF_BLKSIZE and the default density code can be set  with
               MT_ST_DEFDENSITY.  The values for the 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 non-zero, 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_ST_DENSITY_MASK.

       mt_gstat   reports generic  (device  independent)  status  information.
                  The  header  file  defines  macros  for testing these status
                  bits:

                  GMT_EOF(x): The tape is positioned  just  after  a  filemark
                      (always false after an MTSEEK operation).

                  GMT_BOT(x):  The  tape is positioned at the beginning of the
                      first file (always false after an MTSEEK operation).

                  GMT_EOT(x): A tape operation has reached the physical End Of
                      Tape.

                  GMT_SM(x):  The  tape  is  currently positioned at a setmark
                      (always false after an MTSEEK operation).

                  GMT_EOD(x): The tape is positioned at the  end  of  recorded
                      data.

                  GMT_WR_PROT(x):  The  drive  is  write-protected.   For some
                      drives this can  also  mean  that  the  drive  does  not
                      support writing on the current medium type.

                  GMT_ONLINE(x):  The last open(2) found the drive with a tape
                      in place and ready for operation.

                  GMT_D_6250(x), GMT_D_1600(x), GMT_D_800(x):  This  “generic”
                      status  information  reports the current density setting
                      for 9-track 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 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).

   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 */
           };

RETURN VALUE

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

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

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

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

       EIO           The requested operation could not be completed.

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

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

       ENOSYS        Unknown ioctl(2).

       ENXIO         During opening, the tape device does not exist.

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

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

FILES

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

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

NOTES

       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 mode.

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

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

SEE ALSO

       mt(1)

       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
       possibilities.

COLOPHON

       This page is  part  of  release  3.23  of  the  Linux  man-pages
       project.   A  description  of the project, and information about
       reporting bugs, can be found  at  http://www.kernel.org/doc/man-
       pages/.