TM(4)

NAME
tm - SCSI tape controller

SYNOPSIS
device tm0 at ssm? bin 5 maps 512 target 7 unit 0

DESCRIPTION
This device driver supports SCSI target adapters for streaming tape drives. It supports up to eight tape drive units spread across the SSM boards in the system. The SCSI interface for the SSM board supports the target adapter on the SSM board plus seven additional target adapters.

The standard device names begin with ``rtm'' followed by a number representing the tape unit, that is, ``rtm0'' through ``rtm7''. The corresponding no-rewind device name is eight greater than the standard name, that is, ``rtm8'' through ``rtm15''.

IMPLICATIONS OF STREAMING
Because the tm driver supports streaming tapes, the interface to the tm driver is different from that of ``standard'' tape drives, in the following ways:

Block interface
    The block interface is not supported at all. Attempts to use it will return ENXIO.

Raw interface
    The raw tm interface has several differences from standard tape, as described below.

    To provide reasonable performance and drive life, transfers to and from the tape must be done in large chunks. The tm driver assures this by buffering data internally. One consequence of this is that errors on writes often cannot be reported until long after the user passed the data to the driver. Write errors will be reported to the user program on the next write system call after the error occurred, or on the close system call following the error.

    The raw interface does not support block sizes like the standard tape does. The tm tape device has a 512-byte physical block size. User records are packed into these physical blocks on a write and unpacked from these blocks on a read. When reading, the user gets the number of bytes requested, until end-of-file is encountered. If less than a multiple of 512 bytes was written, the file size will be rounded up to a multiple of 512 bytes. The extra bytes contain zeros.

    Reading part-way into a file on the no-rewind device, closing the device, and re-opening it will not work as expected. The tape will always space forward to the beginning of the next file on a close.

IOCTLs
A subset of the ioctl functions are supported at this time. Here is a list of supported and non-supported functions:
    MTFSF	is supported.	(mt fsf nn)
    MTWEOF	is supported.	(mt weof nn)
    MTFSR	is not supported.
    MTBSR	is not supported.
    MTSEOD	is supported.	(mt eod)
    MTREW	is supported.	(mt rewind)
    MTERASE	is supported.	(mt erase)
    MTOFFL	is not supported.
    MTBSF	is not supported.
    MTRET	is supported.	(mt retension)
    MTNORET	is supported.	(mt no retension)

The MTERASE ioctl erases the entire tape and then rewinds it. Reading status (mt status) is not supported.

Ioctls will not always behave as expected if they are intermixed with reads and writes in a program.

OPERATIONAL INFORMATION
A tape is always retensioned when it is first opened for a read or ioctl call. It is erased when it is first opened for a write. Note that if the first operation performed on the tape after insertion (or booting) is ``mt retension'', the tape will be retensioned twice. This automatic retensioning of the tape helps ensure good data transfer, especially on writes to the tape.

The data capacity of a tape will decrease with the number of soft errors on the tape and the number of times the drive must reposition between writes to the tape. These consume tape space. A reasonable expectation for a 450-foot tape is about 44 Mbytes.

The mean time between failure of the tape drive is also affected by the number of repositions while reading or writing the tape. The tape drive will ``stream'' better when it is accessed under a light system load.

CONFIGURATION
The ``tm_bconf'' array in the device driver binary configuration file (/sys/conf/conf_tm.c) must contain an entry for each drive that may be configured; the entries must be in the same order as the ``device'' entries in the kernel configuration file (kernel configuration files are described in the article ``Building DYNIX Systems with Config'' in Volume 2). As installed, ``tm_bconf'' has entries for 4 drives. Superfluous entries are ignored.

TUNING PARAMETERS
The following tuning parameters are defined for each drive in the ``tm_bconf'' array of the binary configuration file:

(1) The buffer size (in K bytes) used to buffer data for I/O transfers. Two buffers, each of this size, are allocated from the kernel's free page pool when the device is opened. These buffers are used to improve tape streaming performance. The buffer space is released when the device is closed.

NOTE: Increasing the size of the data buffers may cause problems with EOT handling on writes. When the driver detects early-EOT from the tape drive, it attempts to write all buffered data to the tape. If the data buffers are sufficiently large, the driver will not be able to write all remaining data to the tape and data will be lost.

(2) Flags for configurable features. The flags available are:

    TMC_OPENFAIL
        When this flag is set, open calls to this device can fail for lack of physical memory for the buffers. When this flag is clear, the buffer memory will always be allocated, but the process may block during this allocation process while other processes are swapped out to free up memory.

    TMC_AUTORET
        When this flag is set, the device will retension automatically on the first access after the tape has been changed. When clear, it will not retension automatically.

    TMC_PRSENSE
        When this flag is set, all SCSI request sense buffers will be printed out, along with the command that caused the sense condition. Otherwise, only sense buffers that are in some way unusual will be printed out, such as media and hardware errors.

    TMC_RSENSE
        When this flag is set, all SCSI request sense buffers associated with SCSI READ operations that caused a sense condition will be printed out.

    TMC_WSENSE
        When this flag is set all SCSI request sense buffers associated with SCSI write and write filemark operations that caused a sense condition will be printed out.

    TMC_SSENSE
        When this flag is set all SCSI request sense buffers associated with SCSI space operations which caused a sense condition will be printed out.

(3) bits: For the ADSI and Emulex target adapters, this should always be a 1 for proper operation of the device.

FILES

/dev/rtm*
    raw files

/sys/conf/conf_tm.c
    binary configuration file

SEE ALSO
mt(1), mtio(4)

DIAGNOSTICS
In the descriptions that follow tmn corresponds to an entry for a drive in the kernel configuration file, where n is the drive number.

tmn: error writing buffer to tape. An I/O error occurred while writing the data to tape.

tmn: error writing file mark on close. An I/O error occurred while writing a file mark to the tape. This message often occurs following the previous message.

tmn: illegal read after write. The process attempted to read data on the tape after successfully completing a write to the tape, without rewinding or retensioning the tape first.

tmn: illegal write after read. The process attempted to write data on the tape after successfully completing a read of the tape, without rewinding or retensioning the tape first.

tmn: bad operation after write. The process attempted to issue an invalid ioctl after successfully completing a write to the tape. Only rewinding and retensioning are valid after a write.

tmn: error on command command; error code= xx; filemark; end of media. Many error messages take this form. The ``error code'' is the device dependent hexadecimal code value for the type of error that occurred. ``filemark'' and ``end of media'' are displayed only if these conditions are indicated by the sense information associated with the error. Error is one of:
    Aborted command
    Hardware error
    Illegal request
    Media error
    Protected Media
    Recoverable error
    Tape not ready
    Unknown error
    Vendor unique error
    Volume overflow
Command is one of:
    erase
    mode select
    read
    request sense
    retension
    rewind
    space
    test unit ready
    write
    write file marks
Some error messages are accompanied by a display of the SCSI command buffer and the sense data buffer at the time of the error.

BUGS
The double file mark at the end of the tape cannot be supported.