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ST(4)                    BSD Kernel Interfaces Manual                    ST(4)

NAME
     st -- SCSI tape driver

SYNOPSIS
     st* at scsibus?
     #st0 at scsibus0 target 4 lun 0 (fixed-configuration example)

DESCRIPTION
     The st driver provides support for SCSI tape drives.  The device can have
     both a raw interface and a block interface; however, only the raw inter-
     face is usually used (or recommended).  The raw interface devices will
     have an 'r' in their names e.g. /dev/rst0.

     SCSI devices have a relatively high level interface and talk to the sys-
     tem via a SCSI adapter and a SCSI adapter driver e.g. ahc(4).  The SCSI
     adapter must be separately configured into the system before a SCSI tape
     can be configured.

     As the SCSI adapter is probed during boot, the SCSI bus is scanned for
     devices.  Any devices found which answer as Sequential type devices will
     be attached to the st driver.

MOUNT SESSIONS
     The st driver is based around the concept of a mount session, which is
     defined as the period between the time that a tape is mounted and the
     time when it is unmounted.  Any parameters set during a mount session
     remain in effect for the remainder of the session or until replaced.  The
     tape can be unmounted, bringing the session to a close in several ways.
     These include:

     1.   Closing an ``unmount device''.

     2.   Using the MTOFFL ioctl(2) command, reachable through the offline
          command of mt(1).

EJECT and REWIND
     Bit 0 of the minor number specifies whether a rewind is attempted when
     the device is closed.  When it is set, the device will not attempt a
     rewind on close and the device will have an 'n' in its name.  For exam-
     ple, /dev/rst0 will rewind on close but /dev/nrst0 will not.

     Bit 1 of the minor number specifies whether an eject is attempted when
     the device is closed.  When it is set, the device will attempt to eject
     its media on close and the device will have an 'e' in its name.  For
     example, /dev/erst0 will eject its media on close but /dev/rst0 will not.

     If both bit 0 and bit 1 are set then an eject will be attempted without a
     rewind and the device will have both an 'e' and an 'n' in its name.  For
     example, /dev/enrst0 will eject its media without first rewinding it on
     close.

     There is no guarantee that the attempted eject or rewind will be sup-
     ported by the actual hardware.

BLOCKING MODES
     SCSI tapes may run in either variable or fixed block-size modes.  Most
     QIC-type devices run in fixed block-size mode, whereas most nine-track
     tapes and many new cartridge formats allow variable block-size.  The dif-
     ference between the two is as follows:

     Variable block-size: Each write made to the device results in a single
     logical record written to the tape.  One can never read or write part of
     a record from tape (though you may request a larger block and read a
     smaller record); nor can one read multiple blocks.  Data from a single
     write is therefore read by a single read.  The block size used may be any
     value supported by the device, the SCSI adapter and the system (usually
     between 1 byte and 64 Kbytes, sometimes more).

     When reading a variable record/block from the tape, the head is logically
     considered to be immediately after the last item read, and before the
     next item after that.  If the next item is a file mark, but it was never
     read, then the next process to read will immediately hit the file mark
     and receive an end-of-file notification.

     Fixed block-size data written by the user is passed to the tape as a suc-
     cession of fixed size blocks.  It may be contiguous in memory, but it is
     considered to be a series of independent blocks.  One may never write an
     amount of data that is not an exact multiple of the blocksize.  One may
     read and write the same data as a different set of records.  In other
     words, blocks that were written together may be read separately, and
     vice-versa.

     If one requests more blocks than remain in the file, the drive will
     encounter the file mark.  Because there is some data to return (unless
     there were no records before the file mark), the read will succeed,
     returning that data.  The next read will return immediately with an EOF.
     (As above, if the file mark is never read, it remains for the next
     process to read if in no-rewind mode.)

FILE MARK HANDLING
     The handling of file marks on write is automatic.  If the user has writ-
     ten to the tape, and has not done a read since the last write, then a
     file mark will be written to the tape when the device is closed.  If a
     rewind is requested after a write, then the driver assumes that the last
     file on the tape has been written, and ensures that there are two file
     marks written to the tape.  The exception to this is that there seems to
     be a standard (which we follow, but don't understand why) that certain
     types of tape do not actually write two file marks to tape, but when
     read, report a ``phantom'' file mark when the last file is read.  These
     devices include the QIC family of devices.  (It might be that this set of
     devices is the same set as that of fixed.  This has not yet been deter-
     mined, and they are treated as separate behaviors by the driver at this
     time.)

IOCTLS
     The following ioctl(2) calls apply to SCSI tapes.  Some also apply to
     other tapes.  They are defined in the header file <sys/mtio.h>.

     MTIOCGET struct mtget *
                Retrieve the status and parameters of the tape.

     MTIOCTOP struct mtop *
                Perform a multiplexed operation.  The argument structure is as
                follows:

                      struct mtop {
                              short   mt_op;
                              int     mt_count;
                      };

                The following operation values are defined for mt_op:

                MTWEOF      Write mt_count end of file marks at the present
                            head position.

                MTFSF       Skip over mt_count file marks.  Leave the head on
                            the EOM side of the last skipped file mark.

                MTBSF       Skip backwards over mt_count file marks.  Leave
                            the head on the BOM (beginning of media) side of
                            the last skipped file mark.

                MTFSR       Skip forwards over mt_count records.

                MTBSR       Skip backwards over mt_count records.

                MTREW       Rewind the device to the beginning of the media.

                MTOFFL      Rewind the media (and, if possible, eject).  Even
                            if the device cannot eject the media it will often
                            no longer respond to normal requests.

                MTNOP       No-op; set status only.

                MTCACHE     Enable controller buffering.

                MTNOCACHE   Disable controller buffering.

                MTSETBSIZ   Set the blocksize to use for the device/mode.  If
                            the device is capable of variable blocksize opera-
                            tion, and the blocksize is set to 0, then the
                            drive will be driven in variable mode.  This
                            parameter is in effect for the present mount ses-
                            sion only.

                MTSETDNSTY  Set the density value (see mt(1)) to use when run-
                            ning in the mode opened (minor bits 2 and 3).
                            This parameter is in effect for the present mount
                            session only.

     MTIOCIEOT  Set end-of-tape processing (not presently supported for st
                devices).

     MTIOCEEOT  Set end-of-tape processing (not presently supported for st
                devices).

FILES
     /dev/[e][n][r]st[0-9]  General form.
     /dev/rst0              No eject, rewind on close.
     /dev/nrst0             No eject, no rewind on close.
     /dev/erst0             Eject, rewind on close.
     /dev/enrst0            Eject, no rewind on close.

SEE ALSO
     chio(1), mt(1), intro(4), mtio(4), scsi(4)

HISTORY
     This st driver was originally written for Mach 2.5 by Julian Elischer,
     and was ported to NetBSD by Charles Hannum.  This man page was edited for
     NetBSD by Jon Buller.

BSD                             March 28, 2017                             BSD