NetBSD/share/man/man4/st.4

.\"	$NetBSD: st.4,v 1.2 1996/10/20 23:15:24 explorer Exp $
.\"
.\" Copyright (c) 1996
.\"     Julian Elischer <julian@freebsd.org>.  All rights reserved.
.\"
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.Dd August 23, 1996
.Dt ST 4
.Os NetBSD
.Sh NAME
.Nm st
.Nd SCSI tape driver
.Sh SYNOPSIS
.Cd st* at scsibus? target ? lun ?
.Cd st1 at scsibus0 target 4 lun 0
.Sh DESCRIPTION
The
.Nm
driver provides support for
.Tn SCSI
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
.Em raw
interface
and a
.Em block
interface; however, only the raw interface is usually used (or
recommended).
.Pp
.Tn SCSI
devices have a relatively high level interface and talk to the
system via a
.Tn SCSI
adapter and a
.Tn SCSI
adapter driver
(e.g.,
.Xr ahc 4 ) .
A
.Tn SCSI
adapter must also be separately configured into the system before a
.Tn SCSI
tape can be configured.
.Pp
As the
.Tn SCSI
adapter is probed during boot, the 
.Tn SCSI
bus is scanned for devices. Any devices found which answer as
.Sq Em Sequential
type devices will be attached to the 
.Nm
driver.
.Sh MOUNT SESSIONS
The 
.Nm
driver is based around the concept of a 
.Dq 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 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 -enum
.It
Closing an `unmount device', referred to as sub-mode 00 below. An
example is
.Pa /dev/rst0 .
.It
Using the MTOFFL
.Xr ioctl 2
command, reachable through the
.Sq Cm offline
command of
.Xr mt 1 .
.It
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.)
.El
.Sh MODES AND SUB-MODES
There are several different 
.Sq 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
.Xr 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
.Dq Em quirk
table in the driver's source code.
.Pp
In addition to the operating modes mentioned above, bits 0 and 1
of the minor number are interpreted as
.Sq sub-modes .
The sub-modes differ in the action taken when the device is closed:
.Bl -tag -width XXXX
.It 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.
.It 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.
.It 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.
.It 11
Reserved.  Currently unused.
.El
.Sh BLOCKING MODES
.Tn SCSI
tapes may run in either 
.Sq Em variable
or
.Sq Em fixed
block-size modes.  Most 
.Tn QIC Ns -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:
.Bl -inset
.It 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 
.Em 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
.Tn SCSI
adapter and the system (usually between 1 byte and 64 Kbytes,
sometimes more).
.Pp
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.
.It 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 vice-versa.
.Pp
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.)
.El
.Sh 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 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.)
.Sh KERNEL CONFIGURATION
Because different tape drives behave differently, there is a
mechanism within the source to
.Nm
to quickly and conveniently recognize and deal with brands and
models of drive that have special requirements.
.Pp
There is a table (called the
.Dq Em 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.
.Sh IOCTLS
The following 
.Xr ioctl 2
calls apply to
.Tn SCSI
tapes.  Some also apply to other tapes.  They are defined in the
header file
.Aq Pa /sys/mtio.h .
.\"
.\" Almost all of this discussion belongs in a separate mt(4)
.\" manual page, since it is common to all magnetic tapes.
.\"
.Pp
.Bl -tag -width MTIOCEEOT
.It Dv MTIOCGET
.Pq Li "struct mtget"
Retrieve the status and parameters of the tape.
.It Dv MTIOCTOP
.Pq Li "struct mtop"
Perform a multiplexed operation.  The argument structure is as follows:
.Bd -literal -offset indent
struct mtop {
	short	mt_op;
	daddr_t	mt_count;
};
.Ed
.Pp
The following operation values are defined for
.Va mt_op :
.Bl -tag -width MTSELDNSTY
.It Dv MTWEOF
Write
.Va mt_count
end of file marks at the present head position.
.It Dv MTFSF
Skip over
.Va mt_count
file marks. Leave the head on the EOM side of the last skipped
file mark.
.It Dv MTBSF
Skip
.Em backwards
over
.Va mt_count
file marks. Leave the head on the BOM (beginning of media)
side of the last skipped file mark.
.It Dv MTFSR
Skip forwards over 
.Va mt_count
records.
.It Dv MTBSR
Skip backwards over
.Va mt_count
records.
.It Dv MTREW
Rewind the device to the beginning of the media.
.It Dv 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 Dv MTNOP
No-op; set status only.
.It Dv MTCACHE
Enable controller buffering.
.It Dv MTNOCACHE
Disable controller buffering.
.It Dv 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.
.It Dv MTSETDNSTY
Set the density value (see 
.Xr 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.
.El
.It Dv MTIOCIEOT
Set end-of-tape processing (not presently supported for
.Nm
devices).
.It Dv MTIOCEEOT
Set end-of-tape processing (not presently supported for
.Nm
devices).
.El
.Sh FILES
.Bl -tag -width /dev/[n][e]rst[0-9] -compact
.It Pa /dev/[n][e]rst[0-9]
general form:
.It Pa /dev/rst0
Mode 0, rewind on close
.It Pa /dev/nrst0
Mode 2, No rewind on close
.It Pa /dev/erst0
Mode 3, Eject on close (if capable)
.El
.Sh DIAGNOSTICS
None.
.Sh SEE ALSO
.Xr mt 1 ,
.Xr scsi 4
.Sh HISTORY
This
.Nm
driver was originally written for
.Tn Mach
2.5 by Julian Elischer, and was ported to 
.Tn NetBSD
by Charles Hannum.  This man page was edited for
.Tn NetBSD
by Jon Buller.