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.Dd August 27, 1993
.Dt ST 4
.Os 386BSD/NetBSD
.Sh NAME
.Nm st
.Nd scsi tape driver
.Sh SYNOPSIS
.Nm device-driver st
.Op Ar count
.Sh DESCRIPTION
The
.Xr st
driver provides support for a 
.Em scsi
tape. It allows the tape
to be run in upto four different modes depending on minor numbers
and supports several different 'sub modes'.
The device can have both a
.Em raw
interface
and a
.Em Block mode
interface however only the raw interface is usually used (or recommended).
In general the interfaces are similar to those described by 
.Xr wt 4 
or
.Xr mt 4 .

.Pp
Where the 
.Xr wt 4
device has a fairly low level interface to the system, 
.Em SCSI
devices have a much higher level interface and talk to the system via
a 
.Em SCSI Adapter
and a
.Em Scsi Adapter driver
e.g. 
.Xr AHA1542 .
A scsi adapter must also be separatly configured into the system
before a scsi tape can be configured.
.Pp
As the scsi adapter is probed during boot, the 
.Em SCSI
bus is scanned for devices. Any devices found which answer as 'Sequential'
type devices will be attached to the 
.Nm
driver. The first found will be attached as
.Em st0
and the next, 
.Em st1
etc.
.Pp
.Sh MOUNT SESSIONS
The 
.Nm
driver is based around the concept of a 
.Em 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 
.Em 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:
.Bl -tag -width ABOUT_THIS_BIG_BUT_REALLY_BIGGER
.It Pa closing an 'unmount device'
This is referred to as sub-mode 00 (see below). An example is /dev/rst0.
.It Pa an MTOFFL ioctl
Reachable throught the 'offline' command of 
.Xr st 1
.It Pa Openning another mode.
Openning 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').
.El
.Pp
Parameters that are required to last across the unmounting of a tape,
should be set on the control device. This is submode 3 (see below) and is
reached through a file with a name of the form /dev/st{y}ctl.{x}, where
{y} is the drive number and {x} is the mode number.
.Pp
.Sh MODES AND SUB MODES
There are four Operation modes. These are  controlled by bits 2
and 3 of the minor number and are designed to allow people 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
.Xr st 1
variant of the
.Xr mt 1
command. When a device corresponding to a particular mode is first mounted,
The operating parameters for that
.Em 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 Operating Mode. To change the parameters for an Operating Mode, 
One must either assign the parameters to the control device, or compile
them into the 'Rogues Gallery' table within the driver.
.Pp
In addition to the four Operation Modes mentionned above, 
bits 0 and 1 of the minor number are interpretted as being 'sub-modes'.
The following sub-modes are supported 
.Bl -tag -width ABOUT_THIS_BIG
.It Pa 00
A close will rewind the device. If the tape has been 
written, then a Filemark will be written before the rewind is requested.
The device is UNMOUNTED.
.It Pa 01
A close will leave the tape MOUNTED.
If the tape was written to a filemark will be written.
No other head positioning takes place.
Any further reads or writes will occur directly after the
last read, or the written filemark.
.It Pa 10
A close will rewind the device. If the tape has been 
written, then a Filemark will be written before the rewind is requested.
On completion of the rewind an UNLOAD command will be issued.
The device is UNMOUNTED.
.It Pa 11
This is a special mode.
It is known as the 
.Em CONTROL DEVICE
for the mode. Parameters set for the mode while in this sub
mode will be remembered from one mount to the next. This allows the
system administrator to set different characteristics (e.g. density,
blocksize, (and eventually compression)) on each mode, and have the
different modes keep those parameters independent of any parameter
changes a user may invoke during a single mount session. At the
completion of the user's mount session, drive parameters will revert
to those set by the administrator. IO operations cannot be performed
on this device/submode. General 
.Xr scsi 4
ioctls 
.Em MUST
be performed against the control device.
.El
.Sh BLOCKING MODES
Scsi Tapes may run in either 'variable' or 'fixed block' modes.
Most QIC type devices run in Fixed block mode, where most 'reel to reel' tapes and 
many new cartridge formats, allow variable blocksize. The difference between
the two is as follows:
.Bl -tag -width variable-blocksize
.It Pa Variable Blocksize
Each write made to the device results in a single logical record
written to the tape. You can never read or write PART of a record
from tape, (though you may request a larger block and read a smaller
record). You cannot read multiple blocks either.  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.  (often variable between 1 byte and 64k (sometimes more)).
.Pp
When reading a variable record/block from the tape, the head is
logically considered to be immediatly after the last item read,
and before the next item after that. If the next item is a Filemark,
but you never read it because you have all the data, then the next
process to read will immediatly read the filemark and return EOF.
(assuming you were in non-rewind mode).
.It Pa fixed Blocksize
Data written by the user is passed to the tape as a succession of
fixed size blocks. It may be contiguouse in ram and read in a single
DMA pass, however it is considered to be a series of independent
blocks. You may never write an amount of data that is not an exact
multiple of the blocksize.  You may read and write the same data
as a different set of records, In other words, blocks that were
written together may be read separatly, and visa versa.
.Pp
If you ask for more blocks than there are left in the file, then
the drive will encounter the filemark. Because there is some data
to return to you (unless there were no records before the filemark)
the driver will return the data to you (less than you requested),
but hide from you the discovery of the Filemark. The  NEXT read
will be returned immediatly with an EOF. If you never Make the next
read, but close the device, the next process to read will immediatly
read the filemark and return EOF. (assuming you were in non-rewind
mode).
.El
.Sh FILEMARK HANDLING
The handling of filemarks on write is pretty much automatic. If you
have written to the tape, and not done a read since, then a filemark will
be written to the tape when you close the device. If a rewind is requested
after a write, then the driver assumes that you have written the last file
on the tape and ensures that there are two filemarks written to the tape.
It takes into account any filemarks already written (whether by close
or by explicit ioctl). 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 filemarks to tape,
but when read, report a 'phantom' filemark 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 detirmined yet, and they are treated as separate behaviours by the
driver at this time.
.Pp
.SH KERNEL CONFIGURATION
In configuring, if an optional
.Ar count
is given in
the specification, that number of scsi tapes are configured;
Most storage for them is allocated only when found so a large number 
of configured devices is cheap. (once the first has included the driver).
.Pp
Because different tape drives behave differently, there is a mechanism 
within the source to st, to quickly and conveniently recognise and deal
with brands and models of drive that have special requirements.
.Pp
There is a table (called the rogues gallery) in which the indentification
strings of known errant drives can be stored. Along with each is
a set of flags that allows the setting of densities and blocksizes for each 
of the 4 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 recognised.
.Pp
.Sh IOCTLS
The following 
.Xr ioctl 2
calls apply to scsi tapes. Some also apply to other tapes. They are defined
in the header file
.Em /sys/mtio.h.

.Bl -tag -width MTIOCEEOT
.It Pa MTIOCGET
Get the mt control structure filled out by the driver, showing
all the present settings.
.It Pa MTIOCTOP
Perform one of the following operations. These operations all have a 
single argument, which is either a boolean, or a signed integer, depending
on the operation.
.Bl -tag -width MTSELDNSTY
.It Pa MTWEOF
Write N end of file marks at the present head position.
.It Pa MTFSF
Skip over N Filemarks. Leave the head on the EOM side of the last skipped
Filemark.
.It Pa MTBSF
Skip BACKWARDS over N Filemarks. Leave the head on the BOM (beginning of media)
side of the last skipped Filemark.
.It Pa MTFSR
Skip forwards over N records.
.It Pa MTBSR
Skip backwards over N records.
.It Pa MTREW
Rewind the device to the beginning of the media.
.It Pa 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.
.It Pa MTNOP
No Op, set status only..
.It Pa MTCACHE
Enable controller Buffering.
.It Pa MTNOCACHE
Disable controller Buffering.
.It Pa 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 set on the control device.
.It Pa MTSETDNSTY
Set the Density value (see 
.Xr st 1
) to use when running in the mode openned (minor bits 2,3).
This parameter is in effect for the present
mount session only, unless set on the control device.
.El
.It Pa MTIOCIEOT
?Set END of TAPE processing... not yet supported.
.It Pa MTIOCEEOT
?Set END of TAPE processing... not yet supported.
.El
.Pp
In addition, The 
.Nm
driver will allow the use of any of the general 
.Xr scsi 4
ioctls, as long as the control device is used.

.Sh FILES
.Bl -tag -width /dev/[n][e]rst[0-9].[0-3] -compact
.It Pa /dev/[n][e]rst[0-9].[0-3]
general form:
.It Pa /dev/rst0.0	
Mode 0, rewind on close
.It Pa /dev/nrst0.2	
Mode 2, No rewind on close
.It Pa /dev/erst0.3
Mode 3, Eject on close (if capable)
.It Pa /dev/rst0	
Another name for rst0.0
.It Pa /dev/nrst0	
Another name for nrst0.0
.It Pa /dev/st0ctl.0	
Parameters set to this device become the default parameters for [en]rst0.0
.It Pa /dev/st0ctl.1	
Parameters set to this device become the default parameters for [en]rst0.1
.It Pa /dev/st0ctl.2	
Parameters set to this device become the default parameters for [en]rst0.2
.It Pa /dev/st0ctl.3	
Parameters set to this device become the default parameters for [en]rst0.3
.El
.Sh DIAGNOSTICS
None.
.Sh SEE ALSO
.Xr mt 1
.Xr st 1
.Sh HISTORY
This
.Nm
driver appeared in MACH 2.5 .