st(4) — Linux manual page


st(4)                   Kernel Interfaces Manual                   st(4)

NAME         top

       st - SCSI tape device

SYNOPSIS         top

       #include <sys/mtio.h>

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

DESCRIPTION         top

       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.  Before Linux 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
       before Linux 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

       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 Linux 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.  Before Linux 2.1.121 allow writes with
       arbitrary byte count if buffering is enabled.  In all other cases
       (before Linux 2.1.121 with buffering disabled or newer kernel)
       the write byte count must be a multiple of the tape block size.

       In Linux 2.6, 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.

              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 files).

              Erase tape.  With Linux 2.6, 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.

              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
              Linux 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.

              Reset drive.

              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

              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 density
              codes supported by a drive can be found from the drive

              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).

              Execute the SCSI unload command (does not eject the tape).

              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 since Linux 2.1).  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 modes

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

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

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

              To control the write threshold the value in mt_count must
              include the constant MT_ST_WRITE_THRESHOLD 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_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 explicitly.

              The Boolean options are:

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

              MT_ST_ASYNC_WRITES (Default: true)
                     When this option is true, write operations return
                     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
                     interchanging tapes with some other systems.

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

              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 Linux 2.6, 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

              MT_ST_SYSV (Default: false)
                     When this option is enabled, the tape devices use
                     the System V 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.,

              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 Linux 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 Linux 2.4.19 and Linux 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;

              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

              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 information.
              The header file defines macros for testing these status

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

                     The tape is positioned at the beginning of the
                     first file (always false after an MTSEEK

                     A tape operation has reached the physical End Of

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

                     The tape is positioned at the end of recorded data.

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

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

                     This “generic” status information reports the
                     current density setting for 9-track ½" tape drives

                     The drive does not have a tape in place.

                     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.

                     The drive has requested cleaning.  Implemented
                     since Linux 2.4.19 and Linux 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 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 */

RETURN VALUE         top

       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 Linux 2.2.18 and
              Linux 2.4.0 the extra bytes have been silently ignored.)

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

       ENOSYS Unknown ioctl(2).

       ENXIO  During opening, the tape device does not exist.

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

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

FILES         top

              the auto-rewind SCSI tape devices

              the nonrewind SCSI tape devices

NOTES         top

       •  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.

       •  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.

       •  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.

       •  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.

       •  The driver's internal buffering allows good throughput in
          fixed-block mode also with small read(2) and write(2) byte
          counts.  With direct transfers this is not possible and may
          cause a surprise when moving to the 2.6 kernel.  The solution
          is to tell the software to use larger transfers (often telling
          it to use larger blocks).  If this is not possible, direct
          transfers can be disabled.

SEE ALSO         top


       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

Linux man-pages (unreleased)     (date)                            st(4)

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