# st

## NAME



           st - SCSI tape device
    


## SYNOPSIS



           **#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



           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 device.
    
           Different options can be specified for the different devices
           within the subgroup of four.  The options take effect when the
           device is opened.  For example, the system administrator can
           define one device that writes in fixed-block mode with a certain
           block size, and one which writes in variable-block mode (if the
           drive supports both modes).
    
           The driver supports **tape partitions** if they are supported by the
           drive.  (Note that the tape partitions have nothing to do with
           disk partitions.  A partitioned tape can be seen as several
           logical tapes within one medium.)  Partition support has to be
           enabled with an ioctl(2).  The tape location is preserved within
           each partition across partition changes.  The partition used for
           subsequent tape operations is selected with an ioctl(2).  The
           partition switch is executed together with the next tape operation
           in order to avoid unnecessary tape movement.  The maximum number
           of partitions on a tape is defined by a compile-time constant
           (originally four).  The driver contains an ioctl(2) that can
           format a tape with either one or two partitions.
    
           Device _/dev/tape_ is usually created as a hard or soft link to the
           default tape device on the system.
    
           Starting from 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.
    
       **Ioctls**
           The driver supports three ioctl(2) requests.  Requests not
           recognized by the **st** driver are passed to the **SCSI** driver.  The
           definitions below are from _/usr/include/linux/mtio.h_ :
    
       **MTIOCTOP — perform a tape operation**
           This request takes an argument of type _(struct mtop *)_.  Not all
           drives support all operations.  The driver returns an **EIO** error if
           the drive rejects an operation.
    
               /* Structure for MTIOCTOP - mag tape op command: */
               struct mtop {
                   short   mt_op;       /* operations defined below */
                   int     mt_count;    /* how many of them */
               };
    
           Magnetic tape operations for normal tape use:
    
           **MTBSF** Backward space over _mt_count_ filemarks.
    
           **MTBSFM** Backward space over _mt_count_ filemarks.  Reposition the
                  tape to the EOT side of the last filemark.
    
           **MTBSR** Backward space over _mt_count_ records (tape blocks).
    
           **MTBSS** Backward space over _mt_count_ setmarks.
    
           **MTCOMPRESSION**
                  Enable compression of tape data within the drive if
                  _mt_count_ is nonzero and disable compression if _mt_count_ is
                  zero.  This command uses the MODE page 15 supported by most
                  DATs.
    
           **MTEOM** Go to the end of the recorded media (for appending files).
    
           **MTERASE**
                  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.
    
           **MTMKPART**
                  Format the tape into one or two partitions.  If _mt_count_ is
                  positive, it gives the size of partition 1 and partition 0
                  contains the rest of the tape.  If _mt_count_ is zero, the
                  tape is formatted into one partition.  From 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.
    
           **MTRESET**
                  Reset drive.
    
           **MTRETEN**
                  Re-tension tape.
    
           **MTREW** Rewind.
    
           **MTSEEK** Seek to the tape block number specified in _mt_count_.  This
                  operation requires either a SCSI-2 drive that supports the
                  **LOCATE** command (device-specific address) or a Tandberg-
                  compatible SCSI-1 drive (Tandberg, Archive Viper, Wangtek,
                  ...).  The block number should be one that was previously
                  returned by **MTIOCPOS** if device-specific addresses are used.
    
           **MTSETBLK**
                  Set the drive's block length to the value specified in
                  _mt_count_.  A block length of zero sets the drive to
                  variable block size mode.
    
           **MTSETDENSITY**
                  Set the tape density to the code in _mt_count_.  The density
                  codes supported by a drive can be found from the drive
                  documentation.
    
           **MTSETPART**
                  The active partition is switched to _mt_count_.  The
                  partitions are numbered from zero.  This command is not
                  allowed for a drive unless the partition support is enabled
                  for the drive (see **MT_ST_CAN_PARTITIONS** below).
    
           **MTUNLOAD**
                  Execute the SCSI unload command (does not eject the tape).
    
           **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
           superuser):
    
           **MTSETDRVBUFFER**
                  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
                  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 different initiators has been successfully
                         written to the medium.
    
                  To control the write threshold the value in _mt_count_ must
                  include the constant **MT_ST_WRITE_THRESHOLD** bitwise ORed
                  with a block count in the low 28 bits.  The block count
                  refers to 1024-byte blocks, not the physical block size on
                  the tape.  The threshold cannot exceed the driver's
                  internal buffer size (see DESCRIPTION, above).
    
                  To set and clear the Boolean options the value in _mt_count_
                  must include one of the constants **MT_ST_BOOLEANS** ,
                  **MT_ST_SETBOOLEANS** , **MT_ST_CLEARBOOLEANS** , or
                  **MT_ST_DEFBOOLEANS** bitwise ORed with whatever combination of
                  the following options is desired.  Using **MT_ST_BOOLEANS** the
                  options can be set to the values defined in the
                  corresponding bits.  With **MT_ST_SETBOOLEANS** the options can
                  be selectively set and with **MT_ST_DEFBOOLEANS** selectively
                  cleared.
    
                  The default options for a tape device are set with
                  **MT_ST_DEFBOOLEANS**.  A nonactive tape device (e.g., device
                  with minor 32 or 160) is activated when the default options
                  for it are defined the first time.  An activated device
                  inherits from the device activated at start-up the options
                  not set 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
                         addresses.
    
                  **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., rewind).
    
                  An example:
    
                      struct mtop mt_cmd;
                      mt_cmd.mt_op = MTSETDRVBUFFER;
                      mt_cmd.mt_count = MT_ST_BOOLEANS |
                              MT_ST_BUFFER_WRITES | MT_ST_ASYNC_WRITES;
                      ioctl(fd, MTIOCTOP, mt_cmd);
    
                  The default block size for a device can be set with
                  **MT_ST_DEF_BLKSIZE** and the default density code can be set
                  with **MT_ST_DEFDENSITY**.  The values for the parameters are
                  or'ed with the operation code.
    
                  With 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;
               };
    
           _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
                  bits:
    
                  **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
                         Tape.
    
                  **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 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 “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 since
                         Linux 2.4.19 and Linux 2.5.43.
    
           _mt_erreg_
                  The only field defined in _mt_erreg_ is the recovered error
                  count in the low 16 bits (as defined by **MT_ST_SOFTERR_SHIFT**
                  and **MT_ST_SOFTERR_MASK**).  Due to inconsistencies in the way
                  drives report recovered errors, this count is often not
                  maintained (most drives do not by default report soft
                  errors but this can be changed with a SCSI MODE SELECT
                  command).
    
           _mt_fileno_
                  reports the current file number (zero-based).  This value
                  is set to -1 when the file number is unknown (e.g., after
                  **MTBSS** or **MTSEEK**).
    
           _mt_blkno_
                  reports the block number (zero-based) within the current
                  file.  This value is set to -1 when the block number is
                  unknown (e.g., after **MTBSF** , **MTBSS** , or **MTSEEK**).
    
       **MTIOCPOS — get tape position**
           This request takes an argument of type _(struct mtpos *)_ and
           reports the drive's notion of the current tape block number, which
           is not the same as _mt_blkno_ returned by **MTIOCGET**.  This drive must
           be a SCSI-2 drive that supports the **READ POSITION** command (device-
           specific address) or a Tandberg-compatible SCSI-1 drive (Tandberg,
           Archive Viper, Wangtek, ... ).
    
               /* structure for MTIOCPOS - mag tape get position command */
               struct mtpos {
                   long mt_blkno;    /* current block number */
               };
    


## RETURN VALUE



           **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.
    
           **EOVERFLOW**
                  An attempt was made to read or write a variable-length
                  block that is larger than the driver's internal buffer.
    
           **EROFS** Open is attempted with **O_WRONLY** or **O_RDWR** when the tape in
                  the drive is write-protected.
    


## FILES



           _/dev/st*_
                  the auto-rewind SCSI tape devices
    
           _/dev/nst*_
                  the nonrewind SCSI tape devices
    


## NOTES



           •  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.
    


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