ST(4)



ST(4)                      Linux Programmer's Manual                     ST(4)

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  detec-
       tion.   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 princi-
       pal (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 us-
       ing  the "no-rewind" device number will not.  (Note that using an auto-
       rewind device for positioning the tape with, for instance, mt does  not
       lead  to  the  desired result: the tape is rewound after the mt command
       and the next command starts from the beginning of the tape).

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

       Devices are typically created by:

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

       There is no corresponding block device.

       The driver uses an internal buffer that has to be large enough to  hold
       at  least one tape block.  In kernels before 2.1.121, the buffer is al-
       located 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  con-
       stant  which  can be overridden with a kernel startup option.  In addi-
       tion to this, the driver tries to allocate a larger temporary buffer at
       run  time if necessary.  However, run-time allocation of large contigu-
       ous 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 ap-
       plies 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  de-
       fault 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 deter-
       mines 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  dis-
       abled.   Kernels  before 2.1.121 allow writes with arbitrary byte count
       if buffering is enabled.  In all other  cases  (kernel  before  2.1.121
       with buffering disabled or newer kernel) the write byte count must be a
       multiple of the tape block size.

       In the 2.6 kernel, the driver tries to use direct transfers between the
       user  buffer and the device.  If this is not possible, the driver's in-
       ternal buffer is used.  The reasons for not using direct transfers  in-
       clude  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 com-
              mand 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 2.6 kernel, short erase (mark tape  empty)  is
              performed if the argument is zero.  Otherwise, long erase (erase
              all) is done.

       MTFSF  Forward space over mt_count filemarks.

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

       MTFSR  Forward space over mt_count records (tape blocks).

       MTFSS  Forward space over mt_count setmarks.

       MTLOAD Execute  the SCSI load command.  A special case is available for
              some   HP   autoloaders.    If   mt_count   is   the    constant
              MT_ST_HPLOADER_OFFSET  plus  a number, the number is sent to the
              drive to control the autoloader.

       MTLOCK Lock the tape drive door.

       MTMKPART
              Format the tape into one or two partitions.  If mt_count is pos-
              itive, it gives the size of partition 1 and partition 0 contains
              the rest of the tape.  If mt_count is zero, the tape is  format-
              ted  into  one  partition.   From kernel version 4.6, a negative
              mt_count specifies the size of partition 0 and the rest  of  the
              tape  contains partition 1.  The physical ordering of partitions
              depends on the drive.  This command is not allowed for  a  drive
              unless  the  partition  support  is  enabled  for the drive (see
              MT_ST_CAN_PARTITIONS below).

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

       MTOFFL Rewind and put the drive off line.

       MTRESET
              Reset drive.

       MTRETEN
              Re-tension tape.

       MTREW  Rewind.

       MTSEEK Seek to the tape block number specified in mt_count.  This oper-
              ation 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  supe-
       ruser):

       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  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 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 in-
              clude 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_DEF-
              BOOLEANS.  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 reli-
                     able multivolume archives.

              MT_ST_ASYNC_WRITES (Default: true)
                     When this option is true, write operations return immedi-
                     ately 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  opera-
                     tion.   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 file-
                     marks 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  di-
                     rectly  to  the drive, potentially speeding up the opera-
                     tion 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  depend-
                     ing  on how the devices are defined.  This option defines
                     when the changes are enforced by the driver  using  SCSI-
                     commands  and when the drives auto-detection capabilities
                     are relied upon.  If this option  is  false,  the  driver
                     sends  the  SCSI-commands  immediately when the device is
                     changed.  If the option is true,  the  SCSI-commands  are
                     not  sent  until a write is requested.  In this case, the
                     drive firmware is allowed to detect  the  tape  structure
                     when  reading and the SCSI-commands are used only to make
                     sure that a tape is  written  according  to  the  correct
                     specification.

              MT_ST_CAN_BSR (Default: false)
                     When  read-ahead  is  used,  the  tape  must sometimes be
                     spaced backward to the correct position when  the  device
                     is  closed  and  the  SCSI command to space backward over
                     records is used for  this  purpose.   Some  older  drives
                     can't  process  this command reliably and this option can
                     be used to instruct the driver not to  use  the  command.
                     The  end  result is that, with read-ahead and fixed-block
                     mode, the tape may not be correctly positioned  within  a
                     file when the device is closed.  With 2.6 kernel, the de-
                     fault is true for drives supporting SCSI-3.

              MT_ST_NO_BLKLIMS (Default: false)
                     Some drives don't accept the READ BLOCK LIMITS SCSI  com-
                     mand.   If this is used, the driver does not use the com-
                     mand.  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 MTIOC-
                     POS  commands  and when changing tape partition).  Other-
                     wise, 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  seman-
                     tics  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 us-
                     ing the device.  In BSD semantics the  tape  position  is
                     not changed.

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

              An example:

                  struct mtop mt_cmd;
                  mt_cmd.mt_op = MTSETDRVBUFFER;
                  mt_cmd.mt_count = MT_ST_BOOLEANS |
                          MT_ST_BUFFER_WRITES | MT_ST_ASYNC_WRITES;
                  ioctl(fd, MTIOCTOP, mt_cmd);

              The  default  block  size  for  a  device  can   be   set   with
              MT_ST_DEF_BLKSIZE  and  the default density code can be set with
              MT_ST_DEFDENSITY.  The values for the parameters are or'ed  with
              the operation code.

              With kernels 2.1.x and later, the timeout values can be set with
              the subcommand MT_ST_SET_TIMEOUT ORed with the timeout  in  sec-
              onds.   The  long  timeout  (used for rewinds and other commands
              that may take a long time) can be set with  MT_ST_SET_LONG_TIME-
              OUT.  The kernel defaults are very long to make sure that a suc-
              cessful command is not timed out with  any  drive.   Because  of
              this,  the  driver may seem stuck even if it is only waiting for
              the timeout.  These commands can be used to set  more  practical
              values  for  a  specific drive.  The timeouts set for one device
              apply for all devices linked to the same drive.

              Starting from kernels 2.4.19 and 2.5.43, the driver  supports  a
              status  bit which indicates whether the drive requests cleaning.
              The method used by the drive to return cleaning  information  is
              set  using  the MT_ST_SEL_CLN subcommand.  If the value is zero,
              the cleaning bit is always zero.   If  the  value  is  one,  the
              TapeAlert  data  defined in the SCSI-3 standard is used (not yet
              implemented).  Values 2-17 are reserved.  If  the  lowest  eight
              bits are >= 18, bits from the extended sense data are used.  The
              bits 9-16 specify a mask to select the bits to look at  and  the
              bits 17-23 specify the bit pattern to look for.  If the bit pat-
              tern 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 de-
              fined  by  MT_ST_BLKSIZE_SHIFT,  MT_ST_BLKSIZE_MASK,  MT_ST_DEN-
              SITY_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" sta-
                  tus information reports  the  current  density  setting  for
                  9-track 1/2" tape drives only.

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

              GMT_IM_REP_EN(x):  Immediate  report  mode.   This bit is set if
                  there are no guarantees that the data  has  been  physically
                  written  to the tape when the write call returns.  It is set
                  zero only when the driver does not buffer data and the drive
                  is set not to buffer data.

              GMT_CLN(x):  The  drive  has requested cleaning.  Implemented in
                  kernels since 2.4.19 and 2.5.43.

       mt_erreg
              The only field defined in mt_erreg is the recovered error  count
              in  the  low  16  bits  (as  defined  by MT_ST_SOFTERR_SHIFT and
              MT_ST_SOFTERR_MASK).  Due to inconsistencies in the  way  drives
              report  recovered  errors,  this  count  is often not maintained
              (most drives do not by default report soft errors but  this  can
              be changed with a SCSI MODE SELECT command).

       mt_fileno
              reports the current file number (zero-based).  This value is set
              to -1 when the file number is unknown (e.g., after MTBSS or  MT-
              SEEK).

       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 allo-
              cate a buffer.

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

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

       EIO    The requested operation could not be completed.

       ENOMEM The byte count in read(2) is  smaller  than  the  next  physical
              block  on  the  tape.   (Before 2.2.18 and 2.4.0 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
       1.  When exchanging data between systems, both systems have to agree on
           the physical tape block size.  The  parameters  of  a  drive  after
           startup  are  often  not  the  ones most operating systems use with
           these devices.  Most systems use drives in variable-block  mode  if
           the  drive supports that mode.  This applies to most modern drives,
           including DATs, 8mm helical scan drives, DLTs, etc.  It may be  ad-
           visable  to  use  these drives in variable-block mode also in Linux
           (i.e., use MTSETBLK or MTSETDEFBLK at system  startup  to  set  the
           mode),  at  least  when exchanging data with a foreign system.  The
           drawback of this is that a fairly large tape block size has  to  be
           used to get acceptable data transfer rates on the SCSI bus.

       2.  Many programs (e.g., tar(1)) allow the user to specify the blocking
           factor on the command line.  Note that this determines the physical
           block size on tape only in variable-block mode.

       3.  In  order  to  use SCSI tape drives, the basic SCSI driver, a SCSI-
           adapter driver and the SCSI tape driver must be  either  configured
           into  the  kernel or loaded as modules.  If the SCSI-tape driver is
           not present, the drive is recognized but the tape support described
           in this page is not available.

       4.  The  driver  writes  error  messages to the console/log.  The SENSE
           codes written into some messages are  automatically  translated  to
           text if verbose SCSI messages are enabled in kernel configuration.

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

SEE ALSO
       mt(1)

       The file drivers/scsi/README.st or Documentation/scsi/st.txt (kernel >=
       2.6)  in the Linux kernel source tree contains the most recent informa-
       tion about the driver and its configuration possibilities

COLOPHON
       This page is part of release 5.07 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
       https://www.kernel.org/doc/man-pages/.

Linux                             2020-04-11                             ST(4)

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