NAME
st —
SCSI/ATAPI tape driver
SYNOPSIS
st* at scsibus? target ? lun ?
st1 at scsibus0 target 4 lun 0
st* at atapibus? drive ? flags 0x0000
DESCRIPTION
The
st driver provides support for SCSI and Advanced
Technology Attachment Packet Interface (ATAPI) 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 and ATAPI devices have a relatively high level interface and talk to the
system via a SCSI or ATAPI adapter and a SCSI or ATAPI adapter driver (e.g.
ahc(4),
pciide(4)). A SCSI or ATAPI
adapter must also be separately configured into the system before a SCSI or
ATAPI tape can be configured.
As the SCSI or ATAPI adapter is probed during boot, the SCSI or ATAPI 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:
- Closing an ‘unmount device’, referred to as
sub-mode 00 below. An example is /dev/rst0.
- Using the
MTOFFL
ioctl(2) command, reachable
through the ‘offline’ command of
mt(1).
- 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).
MODES AND SUB-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
- 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.
BLOCKING MODES
SCSI tapes may run in either ‘
variable’ or
‘
fixed’ block-size modes. Most QIC-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:
- 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 block
devices. This has not been determined yet, and they are treated as separate
behaviors by the driver at this time).
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.
If EARLY WARNING recognitions is
not 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
read(2) system call is set
to −1).
If EARLY WARNING recognition
is 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
not 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.
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.
KERNEL CONFIGURATION
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.
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. Error status and residual is unlatched and
cleared by the driver when it receives this ioctl.
-
-
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.
-
-
MTERASE
- Erase the media from current position. If the field
mt_count is nonzero, a full erase is done (from
current position to end of media). If 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
-
-
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, unless the
device was opened in Control Mode (in which case this set value
persists until a reboot).
-
-
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, 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.
-
-
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.
-
-
MTEWARN
- Enable or disable EARLY WARNING at EOM behaviour (using
the count as a boolean value).
-
-
MTIOCRDSPOS
- (
uint32_t
) Read device logical
block position. Not all drives support this option.
-
-
MTIOCRDHPOS
- (
uint32_t
) Read device hardware
block position. Not all drives support this option.
-
-
MTIOCSLOCATE
- (
uint32_t
) Position the tape to the
specified device logical block position.
-
-
MTIOCHLOCATE
- (
uint32_t
) Position the tape to the
specified hardware block position. Not all drives support this
option.
FILES
- /dev/[n][e]rst[0-9]
- general form:
- /dev/rst0
- Mode 0, Rewind on close
- /dev/nrst0
- Mode 1, No rewind on close
- /dev/erst0
- Mode 2, Eject on close (if capable)
- /dev/enrst0
- Mode 3, Control Mode (elsewise like mode 0)
SEE ALSO
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.
BUGS
The selection of compression could possibly also be usefully done as with a
minor device bit.