ST(4)

NAME
st - SCSI tape device

SYNOPSIS
#include <sys/mtio.h>

int ioctl(int fd, int request [, (void *)arg3])
int ioctl(int fd, MTIOCTOP, (struct mtop *)mt_cmd)
int ioctl(int fd, MTIOCGET, (struct mtget *)mt_status)
int ioctl(int fd, MTIOCPOS, (struct mtpos *)mt_pos)

DESCRIPTION
The st driver provides the interface to a variety of SCSI tape devices. Currently, the driver takes control of all detected devices of type “sequential-access.” The st driver uses major device number 9.

Each device uses two minor device numbers: a principal minor device number, n, assigned sequentially in order of detection, and a “no-rewind” device number, (n+ 128). Devices opened using the principal device number will be sent a REWIND command when they are closed. Devices opened using the “no-rewind” device number will not. Options such as density or block size are not coded in the minor device number. These options must be set by explicit ioctl() calls and remain in effect when the device is closed and reopened.

Devices are typically created by:
    mknod -m 660 /dev/st0 c 9 0
    mknod -m 660 /dev/st1 c 9 1
    mknod -m 660 /dev/nst0 c 9 128
    mknod -m 660 /dev/nst1 c 9 129

There is no corresponding block device. The character device provides buffering and read-ahead by default and supports reads and writes of arbitrary size (limited by the driver's internal buffer size, which defaults to 32768 bytes, but can be changed either by using a kernel startup option or by changing a compile-time constant).

Device /dev/tape is usually created as a hard or soft link to the default tape device on the system.

IOCTLS
The driver supports three ioctl requests. Requests not recognized by the st driver are passed to the scsi driver. The definitions below are from <linux/mtio.h>:

  MTIOCTOP - Perform a tape operation

This request takes an argument of type (struct mtop *). Not all drives support all operations. The driver returns an EIO error if the drive rejects an operation.

/* Structure for MTIOCTOP - mag tape op command: */
struct mtop {
	short	mt_op;		/* operations defined below */
	int	mt_count;	/* how many of them */
};

Magnetic Tape operations:
MTBSF
    Backward space over mt_count filemarks.
MTBSFM
    Backward space over mt_count filemarks. Reposition the tape to the EOT side of the last filemark.
MTBSR
    Backward space over mt_count records (tape blocks).
MTBSS
    Backward space over mt_count setmarks.
MTEOM
    Go to the end of the recorded media (for appending files).
MTERASE
    Erase tape.
MTFSF
    Forward space over mt_count filemarks.
MTFSFM
    Forward space over mt_count filemarks. Reposition the tape to the BOT side of the last filemark.
MTFSR
    Forward space over mt_count records (tape blocks).
MTFSS
    Forward space over mt_count setmarks.
MTNOP
    No op - flushes the driver's buffer as a side effect. Should be used before reading status with MTIOCGET.
MTOFFL
    Rewind and put the drive off line.
MTRESET
    Reset drive.
MTRETEN
    Retension tape.
MTREW
    Rewind.
MTSEEK
    Seek to the tape block number specified in mt_count. This operation requires either a SCSI-2 drive that supports the LOCATE command (device-specific address) or a Tandberg-compatible SCSI-1 drive (Tandberg, Archive Viper, Wangtek, ... ). The block number should be one that was previously returned by MTIOCPOS because the number is device-specific.
MTSETBLK
    Set the drive's block length to the value specified in mt_count. A block length of zero sets the drive to variable block size mode.
MTSETDENSITY
    Set the tape density to the code in mt_count. Some useful density codes are:
    0x00	Implicit	0x11	QIC-525
    0x04	QIC-11	0x12	QIC-1350
    0x05	QIC-24	0x13	DDS
    0x0F	QIC-120	0x14	Exabyte EXB-8200
    0x10	QIC-150	0x15	Exabyte EXB-8500
MTWEOF
    Write mt_count filemarks.
MTWSM
    Write mt_count setmarks.
MTSETDRVBUFFER
    Set various drive and driver options according to bits encoded in mt_count. These consist of the drive's buffering mode, 6 Boolean driver options and the buffer write threshold. These parameters are initialized only when the device is first detected. The settings persist when the device is closed and reopened. A single operation can affect (a) just the buffering mode, (b) just the Boolean options, or (c) just the write threshold.

    A value having zeros in the high-order 4 bits will be used to set the drive's buffering mode. The buffering modes are:

    0   The drive will not report GOOD status on write commands until the data blocks are actually written to the medium.
    1   The drive may report GOOD status on write commands as soon as all the data has been transferred to the drive's internal buffer.
    2   The drive may report GOOD status on write commands as soon as (a) all the data has been transferred to the drive's internal buffer, and (b) all buffered data from different initiators has been successfully written to the medium.

    To control the write threshold the value in mt_count must include the constant MT_ST_WRITE_THRESHOLD logically ORed with a block count in the low 28 bits. The block count refers to 1024-byte blocks, not the physical block size on the tape. The threshold cannot exceed the driver's internal buffer size (see DESCRIPTION, above).

    To set and clear the Boolean options the value in mt_count must include the constant MT_ST_BOOLEANS logically ORed with whatever combination of the following options is desired. Any options not specified will be set false. The Boolean options are:

    MT_ST_BUFFER_WRITES  (Default: true)
        Buffer all write operations. If this option is false and the drive uses a fixed block size, then all write operations must be for a multiple of the block size. This option must be set false to write reliable multi-volume archives.
    MT_ST_ASYNC_WRITES  (Default: true)
        When this options is true write operations return immediately without waiting for the data to be transferred to the drive if the data fits into the driver's buffer. The write threshold determines how full the buffer must be before a new SCSI write command is issued. Any errors reported by the drive will be held until the next operation. This option must be set false to write reliable multi-volume archives.
    MT_ST_READ_AHEAD  (Default: true)
        This option causes the driver to provide read buffering and read-ahead. If this option is false and the drive uses a fixed block size, then all read operations must be for a multiple of the block size.
    MT_ST_TWO_FM  (Default: false)
        This option modifies the driver behavior when a file is closed. The normal action is to write a single filemark. If the option is true the driver will write two filemarks and backspace over the second one.

        Note: This option should not be set true for QIC tape drives since they are unable to overwrite a filemark. These drives detect the end of recorded data by testing for blank tape rather than two consecutive filemarks.

    MT_ST_DEBUGGING  (Default: false)
        This option turns on various debugging messages from the driver (effective only if the driver was compiled with DEBUG defined).
    MT_ST_FAST_EOM  (Default: false)
        This option causes the MTEOM operation to be sent directly to the drive, potentially speeding up the operation but causing the driver to lose track of the current file number normally returned by the MTIOCGET request. If MT_ST_FAST_EOM is false the driver will respond to an MTEOM request by forward spacing over files.
    EXAMPLE
        struct mtop mt_cmd;
        mt_cmd.mt_op = MTSETDRVBUFFER;
        mt_cmd.mt_count = MT_ST_BOOLEANS |
        		MT_ST_BUFFER_WRITES |
        		MT_ST_ASYNC_WRITES;
        ioctl(fd, MTIOCTOP, &mt_cmd);

  MTIOCGET - Get status

This request takes an argument of type (struct mtget *). The driver returns an EIO error if the drive rejects an operation.

/* structure for MTIOCGET - mag tape get status command */
struct mtget {
	long	mt_type;
	long	mt_resid;
	/* the following registers are device dependent */
	long	mt_dsreg;
	long	mt_gstat;
	long	mt_erreg;
	/* The next two fields are not always used */
	daddr_t	mt_fileno;
	daddr_t	mt_blkno;
};

mt_type
    The header file defines many values for mt_type, but the current driver reports only the generic types MT_ISSCSI1 (Generic SCSI-1 tape) and MT_ISSCSI2 (Generic SCSI-2 tape).
mt_resid
    is always zero. (Not implemented for SCSI tape drives.)
mt_dsreg
    reports the drive's current settings for block size (in the low 24 bits) and density (in the high 8 bits). These fields are defined by MT_ST_BLKSIZE_SHIFT, MT_ST_BLKSIZE_MASK, MT_ST_DENSITY_SHIFT, and MT_ST_DENSITY_MASK.
mt_gstat
    reports generic (device independent) status information. The header file defines macros for testing these status bits:
    GMT_EOF(x): The tape is positioned just after a filemark (always false after an MTSEEK operation).
    GMT_BOT(x): The tape is positioned at the beginning of the first file (always false after an MTSEEK operation).
    GMT_EOT(x): A tape operation has reached the physical End Of Tape.
    GMT_SM(x): The tape is currently positioned at a setmark (always false after an MTSEEK operation).
    GMT_EOD(x): The tape is positioned at the end of recorded data.
    GMT_WR_PROT(x): The drive is write-protected. For some drives this can also mean that the drive does not support writing on the current medium type.
    GMT_ONLINE(x): The last open() found the drive with a tape in place and ready for operation.
    GMT_D_6250(x), GMT_D_1600(x), GMT_D_800(x): This “generic” status information reports the current density setting for 9-track ½" tape drives only.
    GMT_DR_OPEN(x): The drive does not have a tape in place.
    GMT_IM_REP_EN(x): Immediate report mode (not supported).
mt_erreg
    The only field defined in mt_erreg is the recovered error count in the low 16 bits (as defined by MT_ST_SOFTERR_SHIFT and MT_ST_SOFTERR_MASK). Due to inconsistencies in the way drives report recovered errors, this count is often not maintained.
mt_fileno
    reports the current file number (zero-based). This value is set to -1 when the file number is unknown (e.g., after MTBSS or MTSEEK).
mt_blkno
    reports the block number (zero-based) within the current file. This value is set to -1 when the block number is unknown (e.g., after MTBSF, MTBSS, or MTSEEK).

  MTIOCPOS - Get tape position

This request takes an argument of type (struct mtpos *) and reports the drive's notion of the current tape block number, which is not the same as mt_blkno returned by MTIOCGET. This drive must be a SCSI-2 drive that supports the READ POSITION command (device-specific address) or a Tandberg-compatible SCSI-1 drive (Tandberg, Archive Viper, Wangtek, ... ).

/* structure for MTIOCPOS - mag tape get position command */
struct	mtpos {
	long 	mt_blkno;	/* current block number */
};

RETURN VALUE

EIO The requested operation could not be completed.

ENOSPC
    A write operation could not be completed because the tape reached end-of-medium.

EACCES
    An attempt was made to write or erase a write-protected tape. (This error is not detected during open().)

ENXIO
    During opening, the tape device does not exist.

EBUSY
    The device is already in use or the driver was unable to allocate a buffer.

EOVERFLOW
    An attempt was made to read or write a variable-length block that is larger than the driver's internal buffer.

EINVAL
    An ioctl() had an illegal argument, or a requested block size was illegal.

ENOSYS
    Unknown ioctl().

COPYRIGHT
Copyright © 1995 Robert K. Nichols.

Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Additional permissions are contained in the header of the source file.

SEE ALSO
mt(1)