MirBSD manpage: st(4)

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


     st - SCSI tape driver


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


     The st driver provides support for SCSI tape drives. It allows a tape
     drive to be run in several different modes depending on minor numbers and
     supports several different "sub-modes". The device can have both a raw
     interface and a block interface; however, only the raw interface is usu-
     ally used (or recommended).

     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)). A SCSI
     adapter must also 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.


     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", referred to as sub-mode 00 below. An
          example is /dev/rst0.

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

     3.   Opening a different mode will implicitly unmount the tape, thereby
          closing off the mode that was previously mounted. All parameters
          will be loaded freshly from the new mode. (See below for more on

     There are several different 'operation' modes. These are controlled by
     bits 2 and 3 of the minor number and are designed to allow users to easi-
     ly read and write different formats of tape on devices that allow multi-
     ple formats. The parameters for each mode can be set individually by hand
     with the mt(1) command. When a device corresponding to a particular mode
     is first mounted, the operating parameters for that mount session are
     copied from that mode. Further changes to the parameters during the ses-
     sion will change those in effect for the session but not those set in the
     operation mode. To change the parameters for an operation mode, one must
     compile them into the "quirk" table in the driver's source code.

     In addition to the operating modes mentioned above, bits 0 and 1 of the
     minor number are interpreted as 'sub-modes'. The sub-modes differ in the
     action taken when the device is closed:

     00    A close will rewind the device; if the tape has been written, then
           a file mark will be written before the rewind is requested. The
           device is unmounted.

     01    A close will leave the tape mounted. If the tape was written to, a
           file mark will be written. No other head positioning takes place.
           Any further reads or writes will occur directly after the last
           read, or the written file mark.

     10    A close will rewind the device. If the tape has been written, then
           a file mark will be written before the rewind is requested. On com-
           pletion of the rewind an unload command will be issued. The device
           is unmounted.

     11    Reserved. Currently unused.


     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
     difference 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 small-
     er 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

     If one requests more blocks than remain in the file, the drive will en-
     counter 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.)


     The handling of file marks on write is automatic. If the user has written
     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, re-
     port a "phantom" file mark when the last file is read. These devices in-
     clude 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 determined, and they
     are treated as separate behaviors by the driver at this time.)


     Because different tape drives behave differently, there is a mechanism
     within the source to st to quickly and conveniently recognize and deal
     with brands and models of drive that have special requirements.

     There is a table (called the "quirk table") in which the identification
     strings of known errant drives can be stored. Alongside each is a set of
     flags that allows the setting of densities and blocksizes for each of the
     modes, along with a set of "QUIRK" flags that can be used to enable or
     disable sections of code within the driver if a particular drive is


     The following ioctl(2) calls apply to SCSI tapes. Some also apply to oth-
     er 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;
                              daddr_t 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 param-
                            eter is in effect for the present mount session

                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 ses-
                            sion only.

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

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


     /dev/[n][e]rst[0-9]  general form
     /dev/rst0            Mode 0, rewind on close
     /dev/nrst0           Mode 2, No rewind on close
     /dev/erst0           Mode 3, Eject on close (if capable)


     mt(1), intro(4), mtio(4), scsi(4), wt(4)


     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.

MirBSD #10-current             August 23, 1996                               3

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