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

       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  detec-
       tion.   In  the 2.6 kernel, the bits above the eight lowermost bits are
       concatenated to the five lowermost bits to form the tape  number.   The
       minor numbers can be grouped into two sets of four numbers: the princi-
       pal (auto-rewind) minor device numbers, n, and the  "no-rewind"  device
       numbers,  (n  + 128).  Devices opened using the principal device number
       will be sent a REWIND command when they  are  closed.   Devices  opened
       using  the  "no-rewind"  device  number  will not.  (Note that using an
       auto-rewind device for positioning the tape with, for instance, mt does
       not  lead  to the desired result: the tape is rewound after the mt com-
       mand and the next command starts from the beginning of the tape).

       Within each group, four minor numbers are available to  define  devices
       with different characteristics (block size, compression, density, etc.)
       When the system starts up, only the first  device  is  available.   The
       other  three are activated when the default characteristics are defined
       (see below).  (By changing compile-time constants, it  is  possible  to
       change  the  balance  between the maximum number of tape drives and the
       number of minor numbers for each drive.  The default allocation  allows
       control  of 32 tape drives.  For instance, it is possible to control up
       to 64 tape drives with two minor numbers for different options.)

       Devices are typically created by:

           mknod -m 666 /dev/st0 c 9 0
           mknod -m 666 /dev/st0l c 9 32
           mknod -m 666 /dev/st0m c 9 64
           mknod -m 666 /dev/st0a c 9 96
           mknod -m 666 /dev/nst0 c 9 128
           mknod -m 666 /dev/nst0l c 9 160
           mknod -m 666 /dev/nst0m c 9 192
           mknod -m 666 /dev/nst0a c 9 224

       There is no corresponding block device.

       The driver uses an internal buffer that has to be large enough to  hold
       at  least  one  tape  block.   In kernels before 2.1.121, the buffer is
       allocated as one contiguous block.  This limits the block size  to  the
       largest  contiguous  block  of memory the kernel allocator can provide.
       The limit is currently 128 kB for 32-bit architectures and  256 kB  for
       64-bit architectures.  In newer kernels the driver allocates the buffer
       in several parts if necessary.  By default, the maximum number of parts
       is  16.   This means that the maximum block size is very large (2 MB if
       allocation of 16 blocks of 128 kB succeeds).

       The driver's internal buffer size is determined by a compile-time  con-
       stant  which  can be overridden with a kernel startup option.  In addi-
       tion to this, the driver tries to allocate a larger temporary buffer at
       run  time if necessary.  However, run-time allocation of large contigu-
       ous blocks of memory may fail and it is advisable not to rely too  much
       on  dynamic  buffer  allocation  with  kernels older than 2.1.121 (this
       applies also to demand-loading the driver with kerneld or kmod).

       The driver does not specifically support any tape drive brand or model.
       After  system start-up the tape device options are defined by the drive
       firmware.  For example, if the drive firmware selects fixed-block mode,
       the tape device uses fixed-block mode.  The options can be changed with
       explicit ioctl(2) calls and remain in effect when the device is  closed
       and reopened.  Setting the options affects both the auto-rewind and the
       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, an
       error is returned.

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

       In the 2.6 kernel, the driver tries to use direct transfers between the
       user  buffer  and  the  device.   If this is not possible, the 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 or more pages of  the  user
       buffer not reachable by the SCSI adapter, and so on.

       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.

       MTFSR         Forward space over mt_count records (tape blocks).

       MTFSS         Forward space over mt_count setmarks.

       MTLOAD        Execute  the SCSI load command.  A special case is avail-
                     able for some HP autoloaders.  If mt_count  is  the  con-
                     stant  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  positive, it gives the size of partition 1 and parti-
                     tion 0 contains the rest of the  tape.   If  mt_count  is
                     zero,  the  tape  is  formatted into one partition.  From
                     kernel version 4.6, a  negative  mt_count  specifies  the
                     size  of  partition  0  and the rest of the tape contains
                     partition 1.  The physical ordering of partitions depends
                     on  the  drive.   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
               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_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 operations return imme-
                      diately 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 reli-
                      able 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

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

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

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

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

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

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

               MT_ST_CAN_BSR (Default: false)
                      When read-ahead is used,  the  tape  must  sometimes  be
                      spaced  backward to the correct position when the device
                      is closed and the SCSI command to  space  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 com-
                      mand.  If this is used, the driver does not use the com-
                      mand.   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 per-
                      forming  the  seek and tell operations (both with MTSEEK
                      and MTIOCPOS commands and when changing tape partition).
                      Otherwise,  the  device-specific addresses are used.  It
                      is highly advisable to set this option if the drive sup-
                      ports  the  logical  addresses  because  they count also
                      filemarks.  There are some drives that support only  the
                      logical block addresses.

               MT_ST_SYSV (Default: false)
                      When  this  option  is enabled, the tape devices use the
                      SystemV semantics.  Otherwise,  the  BSD  semantics  are
                      used.   The most important difference between the seman-
                      tics is what happens when a device used for  reading  is
                      closed: in System V semantics the tape is spaced forward
                      past the next filemark if this has  not  happened  while
                      using the device.  In BSD semantics the tape position is
                      not changed.

               MT_NO_WAIT (Default: false)
                      Enables immediate mode (i.e., 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_TIME-
               OUT.  The kernel defaults are very long to  make  sure  that  a
               successful command is not timed out with any drive.  Because of
               this, the driver may seem stuck even if it is only waiting  for
               the  timeout.  These commands can be used to set more practical
               values for a specific drive.  The timeouts set for  one  device
               apply for all devices linked to the same drive.

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

   MTIOCGET -- get status
       This request takes an argument of type (struct mtget *).

           /* structure for MTIOCGET - mag tape get status command */
           struct mtget {
               long     mt_type;
               long     mt_resid;
               /* the following registers are device dependent */
               long     mt_dsreg;
               long     mt_gstat;
               long     mt_erreg;
               /* The next two fields are not always used */
               daddr_t  mt_fileno;
               daddr_t  mt_blkno;

       mt_type    The  header  file  defines  many values for mt_type, but the
                  current driver reports only  the  generic  types  MT_ISSCSI1
                  (Generic SCSI-1 tape) and MT_ISSCSI2 (Generic SCSI-2 tape).

       mt_resid   contains the current tape partition number.

       mt_dsreg   reports  the drive's current settings for block size (in the
                  low 24 bits) and density (in the high 8 bits).  These fields
                  are   defined  by  MT_ST_BLKSIZE_SHIFT,  MT_ST_BLKSIZE_MASK,
                  MT_ST_DENSITY_SHIFT, and MT_ST_DENSITY_MASK.

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

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

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

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

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

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

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

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

                  GMT_D_6250(x), GMT_D_1600(x), GMT_D_800(x):  This  "generic"
                      status  information  reports the current density setting
                      for 9-track 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  guarantees that the data has been physi-
                      cally written to the tape when the write  call  returns.
                      It is set zero only when the driver does not buffer data
                      and the drive is set not to buffer data.

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

       mt_erreg   The  only  field  defined in mt_erreg is the recovered error
                  count in the low 16 bits (as defined by  MT_ST_SOFTERR_SHIFT
                  and  MT_ST_SOFTERR_MASK.   Due to inconsistencies in the way
                  drives report recovered errors,  this  count  is  often  not
                  maintained (most drives do not by default report soft errors
                  but this can be changed with a SCSI MODE SELECT command).

       mt_fileno  reports the current file number (zero-based).  This value is
                  set to -1 when the file number is unknown (e.g., after MTBSS
                  or MTSEEK).

       mt_blkno   reports the block number  (zero-based)  within  the  current
                  file.   This  value  is  set  to -1 when the block number is
                  unknown (e.g., after MTBSF, MTBSS, or MTSEEK).

   MTIOCPOS -- get tape position
       This request takes an argument of type (struct mtpos *) and reports the
       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 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 physi-
                     cal 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.

              the auto-rewind SCSI tape devices

              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 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.


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

       This  page  is  part of release 4.15 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at

Linux                             2017-09-15                             ST(4)
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