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ST(4)		       FreeBSD 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.

     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 re-
     main 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 ex-
     ample, /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 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, re-
     turning 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 de-
     vices 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 pa-
			    rameter is in effect for the present mount session
			    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 de-
		vices).

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

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.

FreeBSD	13.0		       September 4, 2016		  FreeBSD 13.0

NAME | SYNOPSIS | DESCRIPTION | MOUNT SESSIONS | EJECT and REWIND | BLOCKING MODES | FILE MARK HANDLING | IOCTLS | FILES | SEE ALSO | HISTORY

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