st - Man Page

The driver supports operation in both fixed-block mode and variable-block mode (if supported by the drive). In fixed-block mode the drive writes blocks of the specified size and the block size is not dependent on the byte counts of the write system calls. In variable-block mode one tape block is written for each write call and the byte count determines the size of the corresponding tape block. Note that the blocks on the tape don't contain any information about the writing mode: when reading, the only important thing is to use commands that accept the block sizes on the tape.

In variable-block mode the read byte count does not have to match the tape block size exactly. If the byte count is larger than the next block on tape, the driver returns the data and the function returns the actual block size. If the block size is larger than the byte count, an error is returned.

In fixed-block mode the read byte counts can be arbitrary if buffering is enabled, or a multiple of the tape block size if buffering is disabled. Before Linux 2.1.121 allow writes with arbitrary byte count if buffering is enabled. In all other cases (before Linux 2.1.121 with buffering disabled or newer kernel) the write byte count must be a multiple of the tape block size.

In Linux 2.6, the driver tries to use direct transfers between the user buffer and the device. If this is not possible, the driver's internal buffer is used. The reasons for not using direct transfers include improper alignment of the user buffer (default is 512 bytes but this can be changed by the HBA driver), one or more pages of the user buffer not reachable by the SCSI adapter, and so on.

A filemark is automatically written to tape if the last tape operation before close was a write.

When a filemark is encountered while reading, the following happens. If there are data remaining in the buffer when the filemark is found, the buffered data is returned. The next read returns zero bytes. The following read returns data from the next file. The end of recorded data is signaled by returning zero bytes for two consecutive read calls. The third read returns an error.

The driver supports three ioctl(2) requests. Requests not recognized by the st driver are passed to the SCSI driver. The definitions below are from /usr/include/linux/mtio.h:

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 for normal tape use:

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.

MTCOMPRESSION

Enable compression of tape data within the drive if mt_count is nonzero and disable compression if mt_count is zero. This command uses the MODE page 15 supported by most DATs.

MTEOM

Go to the end of the recorded media (for appending files).

MTERASE

Erase tape. With Linux 2.6, short erase (mark tape empty) is performed if the argument is zero. Otherwise, long erase (erase all) is done.

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.

MTLOAD

Execute the SCSI load command. A special case is available for some HP autoloaders. If mt_count is the constant MT_ST_HPLOADER_OFFSET plus a number, the number is sent to the drive to control the autoloader.

MTLOCK

Lock the tape drive door.

MTMKPART

Format the tape into one or two partitions. If mt_count is positive, it gives the size of partition 1 and partition 0 contains the rest of the tape. If mt_count is zero, the tape is formatted into one partition. From Linux 4.6, a negative mt_count specifies the size of partition 0 and the rest of the tape contains partition 1. The physical ordering of partitions depends on the drive. This command is not allowed for a drive unless the partition support is enabled for the drive (see MT_ST_CAN_PARTITIONS below).

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

Re-tension 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 if device-specific addresses are used.

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. The density codes supported by a drive can be found from the drive documentation.

MTSETPART

The active partition is switched to mt_count. The partitions are numbered from zero. This command is not allowed for a drive unless the partition support is enabled for the drive (see MT_ST_CAN_PARTITIONS below).

MTUNLOAD

Execute the SCSI unload command (does not eject the tape).

MTUNLOCK

Unlock the tape drive door.

MTWEOF

Write mt_count filemarks.

MTWSM

Write mt_count setmarks.

Magnetic tape operations for setting of device options (by the superuser):

MTSETDRVBUFFER

Set various drive and driver options according to bits encoded in mt_count. These consist of the drive's buffering mode, a set of Boolean driver options, the buffer write threshold, defaults for the block size and density, and timeouts (only since Linux 2.1). A single operation can affect only one item in the list below (the Booleans counted as one item.)

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 bitwise 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 one of the constants MT_ST_BOOLEANS, MT_ST_SETBOOLEANS, MT_ST_CLEARBOOLEANS, or MT_ST_DEFBOOLEANS bitwise ORed with whatever combination of the following options is desired. Using MT_ST_BOOLEANS the options can be set to the values defined in the corresponding bits. With MT_ST_SETBOOLEANS the options can be selectively set and with MT_ST_DEFBOOLEANS selectively cleared.

The default options for a tape device are set with MT_ST_DEFBOOLEANS. A nonactive tape device (e.g., device with minor 32 or 160) is activated when the default options for it are defined the first time. An activated device inherits from the device activated at start-up the options not set explicitly.

The Boolean options are:

MT_ST_BUFFER_WRITES (Default: true)

Buffer all write operations in fixed-block mode. 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 multivolume archives.

MT_ST_ASYNC_WRITES (Default: true)

When this option 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 multivolume archives.

MT_ST_READ_AHEAD (Default: true)

This option causes the driver to provide read buffering and read-ahead in fixed-block mode. 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. Most other current drives also detect the end of recorded data and using two filemarks is usually necessary only when interchanging tapes with some other systems.

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 nonzero).

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.

MT_ST_AUTO_LOCK (Default: false)

When this option is true, the drive door is locked when the device file is opened and unlocked when it is closed.

MT_ST_DEF_WRITES (Default: false)

The tape options (block size, mode, compression, etc.) may change when changing from one device linked to a drive to another device linked to the same drive depending on how the devices are defined. This option defines when the changes are enforced by the driver using SCSI-commands and when the drives auto-detection capabilities are relied upon. If this option is false, the driver sends the SCSI-commands immediately when the device is changed. If the option is true, the SCSI-commands are not sent until a write is requested. In this case, the drive firmware is allowed to detect the tape structure when reading and the SCSI-commands are used only to make sure that a tape is written according to the correct specification.

MT_ST_CAN_BSR (Default: false)

When read-ahead is used, the tape must sometimes be spaced backward to the correct position when the device is closed and the SCSI command to space backward over records is used for this purpose. Some older drives can't process this command reliably and this option can be used to instruct the driver not to use the command. The end result is that, with read-ahead and fixed-block mode, the tape may not be correctly positioned within a file when the device is closed. With Linux 2.6, the default is true for drives supporting SCSI-3.

MT_ST_NO_BLKLIMS (Default: false)

Some drives don't accept the READ BLOCK LIMITS SCSI command. If this is used, the driver does not use the command. The drawback is that the driver can't check before sending commands if the selected block size is acceptable to the drive.

MT_ST_CAN_PARTITIONS (Default: false)

This option enables support for several partitions within a tape. The option applies to all devices linked to a drive.

MT_ST_SCSI2LOGICAL (Default: false)

This option instructs the driver to use the logical block addresses defined in the SCSI-2 standard when performing the seek and tell operations (both with MTSEEK and MTIOCPOS commands and when changing tape partition). Otherwise, the device-specific addresses are used. It is highly advisable to set this option if the drive supports the logical addresses because they count also filemarks. There are some drives that support only the logical block addresses.

MT_ST_SYSV (Default: false)

When this option is enabled, the tape devices use the System V semantics. Otherwise, the BSD semantics are used. The most important difference between the semantics is what happens when a device used for reading is closed: in System V semantics the tape is spaced forward past the next filemark if this has not happened while using the device. In BSD semantics the tape position is not changed.

MT_NO_WAIT (Default: false)

Enables immediate mode (i.e., don't wait for the command to finish) for some commands (e.g., rewind).

An 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);

The default block size for a device can be set with MT_ST_DEF_BLKSIZE and the default density code can be set with MT_ST_DEFDENSITY. The values for the parameters are or'ed with the operation code.

With Linux 2.1.x and later, the timeout values can be set with the subcommand MT_ST_SET_TIMEOUT ORed with the timeout in seconds. The long timeout (used for rewinds and other commands that may take a long time) can be set with MT_ST_SET_LONG_TIMEOUT. The kernel defaults are very long to make sure that a successful command is not timed out with any drive. Because of this, the driver may seem stuck even if it is only waiting for the timeout. These commands can be used to set more practical values for a specific drive. The timeouts set for one device apply for all devices linked to the same drive.

Starting from Linux 2.4.19 and Linux 2.5.43, the driver supports a status bit which indicates whether the drive requests cleaning. The method used by the drive to return cleaning information is set using the MT_ST_SEL_CLN subcommand. If the value is zero, the cleaning bit is always zero. If the value is one, the TapeAlert data defined in the SCSI-3 standard is used (not yet implemented). Values 2–17 are reserved. If the lowest eight bits are >= 18, bits from the extended sense data are used. The bits 9–16 specify a mask to select the bits to look at and the bits 17–23 specify the bit pattern to look for. If the bit pattern is zero, one or more bits under the mask indicate the cleaning request. If the pattern is nonzero, the pattern must match the masked sense data byte.

This request takes an argument of type (struct mtget *).

/* 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

contains the current tape partition number.

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(2) 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. This bit is set if there are no guarantees that the data has been physically written to the tape when the write call returns. It is set zero only when the driver does not buffer data and the drive is set not to buffer data.

GMT_CLN(x)

The drive has requested cleaning. Implemented since Linux 2.4.19 and Linux 2.5.43.

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 (most drives do not by default report soft errors but this can be changed with a SCSI MODE SELECT command).

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).

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 */
};