ST(4)

NAME
st - scsi tape driver

SYNOPSIS
tape st
device st1 target 4 lun 0

DESCRIPTION
The st driver provides support for a scsi tape. It allows the tape to be run in upto four different modes depending on minor numbers and supports several different 'sub modes'. The device can have both a raw interface and a Block mode 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. AHA1542. A scsi adapter must also be separatly 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 will be attached as cd0 and the next, cd1 etc. Beginning in 2.1 it is possible to specify what cd 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:

closing an 'unmount device'
    This is referred to as sub-mode 00 (see below). An example is /dev/rst0.

an MTOFFL ioctl
    Reachable throught the 'offline' command of st(1)

Openning another mode.
    Openning 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 submode 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 people 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 st(1) variant of 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 Operating Mode. To change the parameters for an Operating Mode, One must either assign the parameters to the control device, or compile them into the 'Rogues Gallery' table within the driver.

In addition to the four Operation Modes mentionned above, bits 0 and 1 of the minor number are interpretted as being 'sub-modes'. The following sub-modes are supported

00  A close will rewind the device. If the tape has been written, then a Filemark 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 filemark will be written. No other head positioning takes place. Any further reads or writes will occur directly after the last read, or the written filemark.

10  A close will rewind the device. If the tape has been written, then a Filemark 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. It is 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, (and eventually compression)) 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. IO operations cannot be performed on this device/submode. General scsi(4) ioctls MUST be performed against the control device.

BLOCKING MODES
Scsi Tapes may run in either 'variable' or 'fixed block' modes. Most QIC type devices run in Fixed block mode, where most 'reel to reel' tapes and many new cartridge formats, allow variable blocksize. The difference between the two is as follows:

Variable Blocksize
    Each write made to the device results in a single logical record written to the tape. You can never read or write PART of a record from tape, (though you may request a larger block and read a smaller record). You cannot read multiple blocks either. 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. (often variable between 1 byte and 64k (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 Filemark, but you never read it because you have all the data, then the next process to read will immediately read the filemark and return EOF. (assuming you were in non-rewind mode).

fixed Blocksize
    Data written by the user is passed to the tape as a succession of fixed size blocks. It may be contiguouse in ram and read in a single DMA pass, however it is considered to be a series of independent blocks. You may never write an amount of data that is not an exact multiple of the blocksize. You may read and write the same data as a different set of records, In other words, blocks that were written together may be read separatly, and visa versa.

    If you ask for more blocks than there are left in the file, then the drive will encounter the filemark. Because there is some data to return to you (unless there were no records before the filemark) the driver will return the data to you (less than you requested), but hide from you the discovery of the Filemark. The NEXT read will be returned immediately with an EOF. If you never Make the next read, but close the device, the next process to read will immediately read the filemark and return EOF. (assuming you were in non-rewind mode).

FILEMARK HANDLING
The handling of filemarks on write is pretty much automatic. If you have written to the tape, and not done a read since, then a filemark will be written to the tape when you close the device. If a rewind is requested after a write, then the driver assumes that you have written the last file on the tape and ensures that there are two filemarks written to the tape. It takes into account any filemarks already written (whether by close or by explicit ioctl). 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 filemarks to tape, but when read, report a 'phantom' filemark 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 detirmined yet, and they are treated as separate behaviors by the driver at this time.

In configuring, if an optional count is given in the specification, that number of scsi tapes are configured; Most storage for them is allocated only when found so a large number of configured devices is cheap. (once the first has included the driver).

Because different tape drives behave differently, there is a mechanism within the source to st, to quickly and conveniently recognise and deal with brands and models of drive that have special requirements.

There is a table (called the rogues gallery) in which the indentification strings of known errant drives can be stored. Along with each is a set of flags that allows the setting of densities and blocksizes for each of the 4 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 recognised.

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
    Get the mt control structure filled out by the driver, showing all the present settings.

MTIOCTOP
    Perform one of the following operations. These operations all have a single argument, which is either a boolean, or a signed integer, depending on the operation.

    MTWEOF
        Write N end of file marks at the present head position.

    MTFSF
        Skip over N Filemarks. Leave the head on the EOM side of the last skipped Filemark.

    MTBSF
        Skip BACKWARDS over N Filemarks. Leave the head on the BOM (beginning of media) side of the last skipped Filemark.

    MTFSR
        Skip forwards over N records.

    MTBSR
        Skip backwards over N 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 st(1) ) to use when running in the mode openned (minor bits 2,3). This parameter is in effect for the present mount session only, unless set on the control device.

MTIOCIEOT
    ?Set END of TAPE processing... not yet supported.

MTIOCEEOT
    ?Set END of TAPE processing... not yet supported.

In addition, The st driver will allow the use of any of the general scsi(4) ioctls, as long as the control device is used.

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) st(1)

HISTORY
This st driver appeared in MACH 2.5 .