ST(4)

NAME
st - SCSI tape driver

SYNOPSIS
tape st
device st1 at scbus0 target 4 lun 0

DESCRIPTION
The st driver provides support for a SCSI tape. It allows the tape to be run in up to four 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). In general the interfaces are similar to those described by wt(4) or mt(4).

Where the wt(4) device has a fairly low level interface to the system, SCSI devices have a much higher 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. In FreeBSD releases prior to 2.1, the first found is attached as st0 and the next, st1, etc. Beginning in FreeBSD 2.1 it is possible to specify what st unit a device should come on line as; refer to scsi(4) for details on kernel configuration.

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', 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 st(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.)

Parameters that are required to last across the unmounting of a tape should be set on the control device. This is sub-mode 3 (see below) and is reached through a file with a name of the form /dev/st Y ctl. X, where Y is the drive number and X is the mode number.

MODES AND SUB-MODES
There are four ‘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 either assign the parameters to the control device, or compile them into the “Rogues Gallery” table in the driver's source code.

In addition to the four 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 a special mode, known as the “control device” for the mode. Parameters set for the mode while in this sub-mode will be remembered from one mount to the next. This allows the system administrator to set different characteristics (e.g., density, blocksize) on each mode, and have the different modes keep those parameters independent of any parameter changes a user may invoke during a single mount session. At the completion of the user's mount session, drive parameters will revert to those set by the administrator. I/O operations cannot be performed on this device/sub-mode. General scsi(4) ioctls can only be performed against the control device.

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.)

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 “Rogues Gallery”) 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 four 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.

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, unless set on the control device.

    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 set on the control device.

MTIOCIEOT
    Set end-of-tape processing (not presently supported for st devices).

MTIOCEEOT
    Set end-of-tape processing (not presently supported for st devices).

In addition, the st driver will allow the use of any of the general scsi(4) ioctl() commands, on the control device only.

FILES
/dev/[n][e]rst[0-9].[0-3]
    general form:
/dev/rst0.0
    Mode 0, rewind on close
/dev/nrst0.2
    Mode 2, No rewind on close
/dev/erst0.3
    Mode 3, Eject on close (if capable)
/dev/rst0
    Another name for rst0.0
/dev/nrst0
    Another name for nrst0.0
/dev/st0ctl.0
    Parameters set to this device become the default parameters for [en]rst0.0
/dev/st0ctl.1
    Parameters set to this device become the default parameters for [en]rst0.1
/dev/st0ctl.2
    Parameters set to this device become the default parameters for [en]rst0.2
/dev/st0ctl.3
    Parameters set to this device become the default parameters for [en]rst0.3

DIAGNOSTICS
None.

SEE ALSO
mt(1), scsi(4), st(9)

HISTORY
This st driver was originally written for Mach 2.5, and was ported to FreeBSD by Julian Elischer.