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

     st - SCSI tape driver

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

     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
     interface 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
     system 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.

     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).

     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
     example, /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

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

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

     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;
			      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
			    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.

		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.

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

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

     /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.

     chio(1), 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.

OpenBSD 4.9		       November 2, 2007			   OpenBSD 4.9

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