ST(4) OpenBSD Programmer's Manual ST(4)NAMEst - SCSI tape driver
SYNOPSIS
st* at scsibus?
#st0 at scsibus0 target 4 lun 0 (fixed-configuration example)
DESCRIPTION
The st driver provides support for SCSI tape drives. The device can have
both a raw interface and a block interface; however, only the raw
interface is usually used (or recommended). The raw interface devices
will have an `r' in their names e.g. /dev/rst0.
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). The
SCSI adapter must 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.
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''.
2. Using the MTOFFL ioctl(2) command, reachable through the offline
command of mt(1).
EJECT and REWIND
Bit 0 of the minor number specifies whether a rewind is attempted when
the device is closed. When it is set, the device will not attempt a
rewind on close and the device will have an `n' in its name. For
example, /dev/rst0 will rewind on close but /dev/nrst0 will not.
Bit 1 of the minor number specifies whether an eject is attempted when
the device is closed. When it is set, the device will attempt to eject
its media on close and the device will have an `e' in its name. For
example, /dev/erst0 will eject its media on close but /dev/rst0 will not.
If both bit 0 and bit 1 are set then an eject will be attempted without a
rewind and the device will have both an `e' and an `n' in its name. For
example, /dev/enrst0 will eject its media without first rewinding it on
close.
There is no guarantee that the attempted eject or rewind will be
supported by the actual hardware.
BLOCKING MODES
SCSI tapes may run in either variable or fixed block-size modes. Most
QIC-type devices run in fixed block-size mode, whereas 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 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.)
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. This has not yet been
determined, and they are treated as separate behaviors by the driver at
this time.)
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.
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).
FILES
/dev/[e][n][r]st[0-9] General form.
/dev/rst0 No eject, rewind on close.
/dev/nrst0 No eject, no rewind on close.
/dev/erst0 Eject, rewind on close.
/dev/enrst0 Eject, no rewind on close.
SEE ALSOchio(1), 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.
OpenBSD 4.9 November 2, 2007 OpenBSD 4.9
