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NetBSD 6.1.5 - man page for st (netbsd section 4)

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

NAME
     st -- SCSI/ATAPI tape driver

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

DESCRIPTION
     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 recom-
     mended).

     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 configured.

     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.

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', 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 modes).

MODES AND SUB-MODES
     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 indi-
     vidually 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 com-
	   mand 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 block-
	   size) 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.

BLOCKING MODES
     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 sup-
     ported 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 inde-
     pendent 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 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 writ-
     ten to the tape.  The exception to this is that there seems to be a standard (which we fol-
     low, 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).

EOM HANDLING
     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 applica-
     tion 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 pre-
     vious 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.

KERNEL CONFIGURATION
     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
     recognized.

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. 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 fol-
		lows:

		      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 can-
			    not 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 capa-
			    ble 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 BOT.

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

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

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

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

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

FILES
     /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)

SEE ALSO
     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.

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

BSD					 August 23, 1996				      BSD
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