st
—
SCSI/ATAPI tape driver
st* at scsibus? target ? lun ?
st1 at scsibus0 target 4 lun 0
st* at atapibus? drive ? flags 0x0000
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.
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).
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.
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).
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).
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.
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. 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.
- /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)
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.
The selection of compression could possibly also be usefully done as with a
minor device bit.