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 MTOFFLioctl(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)
Thisst 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.