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, 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
   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 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
                 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  positive,  it  gives  the  size  of  partition  1 and
                 partition 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
                 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

           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

           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
                  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
                  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

           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_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
                  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 " 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).

   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
                 physical  block  on  the tape.  (Before 2.2.18 and
                 2.4.0-test6 the extra  bytes  have  been  silently

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

   ENOSYS        Unknown ioctl(2).

   ENXIO         During opening, the tape device does not exist.

   EOVERFLOW     An attempt was made to read or write  a  variable-
                 length  block  that  is  larger  than the 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

   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  information about the driver and its configuration


   This page is  part  of  release  4.09  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


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