Data Transfer¶

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, the requested amount of data from the start of the block is returned and the rest of the block is discarded.

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. Kernels before 2.1.121 allow writes with arbitrary byte count if buffering is enabled. In all other cases (kernel before 2.1.121 with buffering disabled or newer kernel) the write byte count must be a multiple of the tape block size.

In the 2.6 kernel, 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 of more pages of the user buffer not reachable by the SCSI adapter, etc.

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.

Ioctls¶

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:

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 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 non-zero 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 2.6 kernel, 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 non-zero, it gives the size of the first partition and the second partition contains the rest of the tape. If mt_count is zero, the tape is formatted into one partition. 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 in kernels 2.1 and later). A single operation can affect only one item in the list above (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:

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 one of the constants MT_ST_BOOLEANS, MT_ST_SETBOOLEANS, MT_ST_CLEARBOOLEANS, or MT_ST_DEFBOOLEANS logically or'ed 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 non-active 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:

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 kernels 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 kernels 2.4.19 and 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 non-zero, the pattern must match the masked sense data byte.

MTIOCGET — Get status¶

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:

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).

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 */
};