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       st - SCSI tape device


       #include <sys/mtio.h>

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


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

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

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

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

       There is no corresponding block device.

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

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

       The driver does not specifically support any tape drive brand or model.
       After system start-up the tape device options are defined by the  drive
       firmware.  For example, if the drive firmware selects fixed-block mode,
       the tape device uses fixed-block mode. The options can be changed  with
       explicit  ioctl()  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().  The tape
       location  is  preserved within each partition across partition changes.
       The partition used for subsequent tape operations is selected  with  an
       ioctl().   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()  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.


       The driver supports operation in both fixed-block  mode  and  variable-
       block  mode (if supported by the drive).  In fixed-block mode the drive
       writes blocks of the specified size and the block size is not dependent
       on  the  byte counts of the write system calls.  In variable-block mode
       one tape block is written for  each  write  call  and  the  byte  count
       determines  the  size  of  the  corresponding tape block. Note that the
       blocks on the tape don’t contain  any  information  about  the  writing
       mode:  when  reading,  the only important thing is to use commands that
       accept the block sizes on the tape.

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

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

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

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

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


       The driver supports three ioctl() 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
       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       Retension 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).  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 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

               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
               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 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
               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  SYSV  semantics  the tape is spaced forward past the
                      next filemark if this has not happened while  using  the
                      device.  In  BSD  semantics  the  tape  position  is not
               MT_NO_WAIT (Default: false)
                      Enables immediate mode (i.e., don’t wait for the command
                      to finish) for some commands (e.g., rewind).
                      struct mtop mt_cmd;
                      mt_cmd.mt_op = MTSETDRVBUFFER;
                      mt_cmd.mt_count = MT_ST_BOOLEANS |
                              MT_ST_BUFFER_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
                  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() 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 >= 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 */


       EIO           The requested operation could not be completed.

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

       ENOMEM        The byte  count  in  read()  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.)

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

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

       ENXIO         During opening, the tape device does not exist.

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

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

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

       ENOSYS        Unknown ioctl().

       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 non-rewind SCSI tape devices


       The  driver  has  been  written by Kai Mäkisara (
       starting from a driver written by Dwayne Forsyth. Several other  people
       have also contributed to the driver.



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


       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) 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() and write() 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.


       Copyright © 1995 Robert K. Nichols.
       Copyright © 1999-2005 Kai Mäkisara.

       Permission is granted to make and distribute verbatim  copies  of  this
       manual  provided  the  copyright  notice and this permission notice are
       preserved on all copies.  Additional permissions are contained  in  the
       header of the source file.