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ST(4)                        Device Drivers Manual                       ST(4)

     st - SCSI/ATAPI tape driver

     st* at scsibus? target ? lun ?
     st1 at scsibus0 target 4 lun 0
     st* at atapibus? drive ? flags 0x0000

     The st driver provides support for SCSI and Advanced Technology
     Attachment Packet Interface (ATAPI) 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
     usually used (or recommended).

     SCSI and ATAPI devices have a relatively high level interface and talk to
     the system via a SCSI or ATAPI adapter and a SCSI or ATAPI adapter driver
     (e.g.  ahc(4), pciide(4)).  A SCSI or ATAPI adapter must also be
     separately configured into the system before a SCSI or ATAPI tape can be

     As the SCSI or ATAPI adapter is probed during boot, the SCSI or ATAPI 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
     easily read and write different formats of tape on devices that allow
     multiple 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 session 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
           completion of the rewind an unload command will be issued.  The
           device is unmounted.

     11    This is Control mode, which  allows the tape driver to be opened
           without a tape inserted to allow various ioctls (e.g. MTIOCGET or
           MTIOCTOP to set density or blocksize) and raw SCSI command on
           through. I/O can be done in this mode, if desired, with the same
           rewind/eject behaviour as mode 01. This isn't really an 'action
           taken on close' type of distinction, but this seems to be the place
           to put this mode.

     SCSI tapes may run in either `variable' or `fixed' block-size modes.
     Most QIC-type devices run in fixed block-size mode, where 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
     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
     succession 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
     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).

     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, 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 block devices.  This has not
     been determined yet, and they are treated as separate behaviors by the
     driver at this time).

     Attempts to write past EOM and how EOM is reported are handled slightly
     differently based upon whether EARLY WARNING recognition is enabled in
     the driver.

     If EARLY WARNING recognitions is not enabled, then detection of EOM (as
     reported in SCSI Sense Data with an EOM indicator) causes the write
     operation to be flagged with I/O error (EIO).  This has the effect for
     the user application of not knowing actually how many bytes were read
     (since the return of the read(2) system call is set to -1).

     If EARLY WARNING recognition is enabled, then detection of EOM (as
     reported in SCSI Sense Data with an EOM indicator) has no immediate
     effect except that the driver notes that EOM has been detected. If the
     write completing didn't transfer all data that was requested, then the
     residual count (counting bytes not written) is returned to the user
     application. In any event, the next attempt to write (if that is the next
     action the user application takes) is immediately completed with no data
     transferred, and a residual returned to the user application indicating
     that no data was transferred.  This is the traditional UNIX EOF
     indication. The state that EOM had been seen is then cleared.

     In either mode of operation, the driver does not prohibit the user
     application from writing more data, if it chooses to do so. This will
     continue up until the physical end of media, which is usually signalled
     internally to the driver as a CHECK CONDITION with the Sense Key set to
     VOLUME OVERFLOW. When this or any otherwise unhandled error occurs, an
     error return of EIO will be transmitted to the user application.  This
     does indeed mean that if EARLY WARNING is enables and the device
     continues to set EOM indicators prior to hitting physical end of media,
     that an indeterminate number of 'short write returns' as described in the
     previous paragraph will occur. However, the expected user application
     behaviour (in common with other systems) is to close the tape and rewind
     and request another tape upon the receipt of the first EOM indicator,
     possibly after writing one trailer record.

     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
     other tapes.  They are defined in the header file <sys/mtio.h>.

     MTIOCGET   (struct mtget) Retrieve the status and parameters of the tape.
                Error status and residual is unlatched and cleared by the
                driver when it receives this ioctl.

     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.

                MTERASE     Erase the media from current position. If the
                            field mt_count is nonzero, a full erase is done
                            (from current position to end of media). If
                            mt_count is zero, only an erase gap is written. It
                            is hard to say which drives support only one but
                            not the other option

                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
                            operation, 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
                            session only, unless the device was opened in
                            Control Mode (in which case this set value
                            persists until a reboot).

                MTSETDNSTY  Set the density value (see mt(1)) to use when
                            running in the mode opened (minor bits 2 and 3).
                            This parameter is in effect for the present mount
                            session only, unless the device was opened in
                            Control Mode (in which case this set value
                            persists until a reboot).  Any byte sized value
                            may be specified. Note that only a very small
                            number of them will actually usefully work. The
                            rest will cause the tape drive to spit up.

                MTCMPRESS   Enable or disable tape drive data compression.
                            Typically tape drives will quite contentedly
                            ignore settings on reads, and will probably keep
                            you from changing density for writing anywhere but

                MTEWARN     Enable or disable EARLY WARNING at EOM behaviour
                            (using the count as a boolean value).

                (uint32_t) Read device logical block position.  Not all drives
                support this option.

                (uint32_t) Read device hardware block position.  Not all
                drives support this option.

                (uint32_t) Position the tape to the specified device logical
                block position.

                (uint32_t) Position the tape to the specified hardware block
                position.  Not all drives support this option.

     /dev/[n][e]rst[0-9]  general form:
     /dev/rst0            Mode 0, Rewind on close
     /dev/nrst0           Mode 1, No rewind on close
     /dev/erst0           Mode 2, Eject on close (if capable)
     /dev/enrst0          Mode 3, Control Mode (elsewise like mode 0)

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

     The selection of compression could possibly also be usefully done as with
     a minor device bit.

NetBSD 10.99                    August 23, 1996                   NetBSD 10.99