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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 lqsequential-accessrq.  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 lower-
          most 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 lqno-rewindrq 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 lqno-rewindrq device number
          will not.  (Note that using an auto-rewind device for posi-
          tioning 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 acti-
          vated 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

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              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

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          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

          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 pos-
          sible, 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.

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          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:

               Backward space over mt_count filemarks.

               Backward space over mt_count filemarks.  Reposition the
               tape to the EOT side of the last filemark.

               Backward space over mt_count records (tape blocks).

               Backward space over mt_count setmarks.

               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.

               Go to the end of the recorded media (for appending

               Erase tape.  With 2.6 kernel, short erase (mark tape
               empty) is performed if the argument is zero.

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               Otherwise, long erase (erase all) is done.

               Forward space over mt_count filemarks.

               Forward space over mt_count filemarks.  Reposition the
               tape to the BOT side of the last filemark.

               Forward space over mt_count records (tape blocks).

               Forward space over mt_count setmarks.

               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 num-
               ber is sent to the drive to control the autoloader.

               Lock the tape drive door.

               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 parti-
               tion.  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).

               No op-flushes the driver's buffer as a side effect.
               Should be used before reading status with MTIOCGET.

               Rewind and put the drive off line.

               Reset drive.

               Re-tension tape.


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               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-
               specific addresses are used.

               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.

               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.

               The active partition is switched to mt_count. The par-
               titions 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).

               Execute the SCSI unload command (does not eject the

               Unlock the tape drive door.

               Write mt_count filemarks.

               Write mt_count setmarks.

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

               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
               below (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

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               modes are:

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

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

                      2   The drive may report GOOD status on write
                          commands as soon as (a) all the data has
                          been transferred to the drive's internal
                          buffer, and (b) all buffered data from dif-
                          ferent 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,

               To set and clear the Boolean options the value in
               mt_count must include one of the constants
               or MT_ST_DEFBOOLEANS bitwise ORed with whatever combi-
               nation 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 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)

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                    When this option is true, write operations return
                    immediately without waiting for the data to be
                    transferred to the drive if the data fits into the
                    driver's buffer.  The write threshold determines
                    how full the buffer must be before a new SCSI
                    write command is issued.  Any errors reported by
                    the drive will be held until the next operation.
                    This option must be set false to write reliable
                    multivolume archives.

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

               MT_ST_TWO_FM (Default: false)
                    This option modifies the driver behavior when a
                    file is closed.  The normal action is to write a
                    single filemark.  If the option is true, the
                    driver will write two filemarks and backspace over
                    the second 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 inter-
                    changing tapes with some other systems.

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

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

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

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               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

               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 reli-
                    ably 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 logi-
                    cal block addresses defined in the SCSI-2 standard
                    when performing the seek and tell operations (both
                    with MTSEEK and MTIOCPOS commands and when chang-
                    ing tape partition).  Otherwise, the device-
                    specific addresses are used.  It is highly advis-
                    able to set this option if the drive supports the
                    logical addresses because they count also

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                    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 System V semantics.  Otherwise, the BSD seman-
                    tics are used.  The most important difference
                    between the semantics is what happens when a
                    device used for reading is closed: in System V
                    semantics the tape is spaced forward past the next
                    filemark if this has not happened while using the
                    device.  In BSD semantics the tape position is not

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

               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 parame-
               ters 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

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               (not yet implemented).  Values 2en17 are reserved.  If
               the lowest eight bits are >= 18, bits from the extended
               sense data are used.  The bits 9en16 specify a mask to
               select the bits to look at and the bits 17en23 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 pat-
               tern 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;

               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 (Gen-
               eric SCSI-2 tape).

               contains the current tape partition number.

               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,

               reports generic (device independent) status informa-
               tion.  The header file defines macros for testing these
               status bits:

               GMT_EOF(x): The tape is positioned just after a file-
                   mark (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 opera-

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

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

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

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

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

               GMT_D_6250(x), GMT_D_1600(x), GMT_D_800(x): This lqgen-
                   ericrq status information reports the current den-
                   sity setting for 9-track 1/2" tape drives only.

               GMT_DR_OPEN(x): The drive does not have a tape in

               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.  Imple-
                   mented in kernels since 2.4.19 and 2.5.43.

               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).

               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).

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

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        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 */

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

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

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

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

          EIO  The requested operation could not be completed.

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

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

               Unknown ioctl(2).

               During opening, the tape device does not exist.

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

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               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 parame-
              ters 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 exchang-
              ing 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 automati-
              cally 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.


     Page 14                       Linux             (printed 5/24/22)

     ST(4)                     (2020-04-11)                      ST(4)


          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 configura-
          tion possibilities

          This page is part of release 5.10 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

     Page 15                       Linux             (printed 5/24/22)