man/man4/st.4

.\"	$NetBSD: st.4,v 1.15 1999/03/16 01:19:18 garbled Exp $
.\"
.\" Copyright (c) 1996
.\"     Julian Elischer <julian@freebsd.org>.  All rights reserved.
.\"
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.Dd August 23, 1996
.Dt ST 4
.Os
.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
.Sq 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
.Sq unmount device ,
referred to as sub-mode 00 below.
An example is
.Pa /dev/rst0 .
.It
Using the
.Dv 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
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 blocksize) 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.
.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
.Sq 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 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.
.Pp
If EARLY WARNING recognitions is \fBnot\fR 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
.Xr read 2
system call is set to \(mi1).
.Pp
If EARLY WARNING recognition \fBis\fR 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 \fBnot\fR 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 application indicating that no data was transferred.
This is the traditional UNIX EOF indication. The state that EOM had
been seen is then cleared.
.Pp
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 previous 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.
.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. Error status
and residual is unlatched and cleared by the driver when it receives
this ioctl.
.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 MTERASE
Erase the media from current position. If the field
.Va mt_count
is nonzero, a full erase is done (from current position to end of
media). If
.Va 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
.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, unless
the device was opened in Control Mode (in which case this set value persists
until a reboot).
.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, 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.
.It Dv 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.
.It Dv MTEWARN
Enable or disable EARLY WARNING at EOM behaviour (using the count
as a boolean value).
.El
.It Dv MTIOCRDSPOS
.Pq Li "u_int32_t"
Read device logical block position.
Not all drives support this option.
.It Dv MTIOCRDHPOS
.Pq Li "u_int32_t"
Read device hardware block position.
Not all drives support this option.
.It Dv MTIOCSLOCATE
.Pq Li "u_int32_t"
Position the tape to the specified device logical block position.
.It Dv MTIOCHLOCATE
.Pq Li "u_int32_t"
Position the tape to the specified hardware block position.
Not all drives support this option.
.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 1, No rewind on close
.It Pa /dev/erst0
Mode 2, Eject on close (if capable)
.It Pa /dev/enst0
Mode 3, Control Mode (elsewise like mode 0)
.El
.Sh BUGS
The selection of compression could possibly also be usefully done
as with a minor device bit.
.Sh DIAGNOSTICS
A variety, far too many to list.
.Sh SEE ALSO
.Xr mt 1 ,
.Xr intro 4 ,
.Xr mtio 4 ,
.Xr scsi 4
.Sh HISTORY
This
.Nm
driver was originally written for
.Tn Mach
2.5 by Julian Elischer, and was ported to
.Nx
by Charles Hannum.
This man page was edited for
.Nx
by Jon Buller.