st - SCSI tape driver
st* at scsibus? target ? lun ?
#st0 at scsibus0 target 4 lun 0 (fixed-configuration example)
The st driver provides support for 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 raw
interface and a block interface; however, only the raw interface is usually
used (or recommended).
SCSI devices have a relatively high level interface and talk
to the system
via a SCSI adapter and a SCSI adapter driver (e.g.,
ahc(4)). A SCSI
adapter must also 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.
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.)
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 individually 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 command will be issued. The device
is unmounted.
11 Reserved. Currently unused.
SCSI tapes may run in either `variable' or `fixed' blocksize modes.
Most QIC-type devices run in fixed block-size mode, whereas
most ninetrack
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 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 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.
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.)
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. This has not yet
been determined,
and they are treated as separate behaviors by the
driver at this
time.)
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.
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;
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 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 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.
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.
MTIOCIEOT Set end-of-tape processing (not presently supported for st devices).
MTIOCEEOT Set end-of-tape processing (not presently supported for st devices).
/dev/[n][e]rst[0-9] general form
/dev/rst0 Mode 0, rewind on close
/dev/nrst0 Mode 2, No rewind on close
/dev/erst0 Mode 3, Eject on close (if capable)
mt(1), intro(4), mtio(4), scsi(4), wt(4)
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.
OpenBSD 3.6 August 23, 1996
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