st: Páskové zařízení SCSI

Originální popis anglicky: st - SCSI tape device

Návod, kniha: Linux Programmer's Manual

STRUČNĚ

#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);

POPIS / INSTRUKCE

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 eight minor device numbers. The lower-most five bits in the minor numbers are assigned sequentially in the order of detection. The minor numbers can be grouped into two sets of four numbers: the principal (auto-rewind) minor device numbers, n, and a “no-rewind” device numbers, (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. (Note that using an auto-rewind device for positioning the tape with, for instance, mt does not lead to the desired result: the tape is rewound after the mt command and the next command starts from the beginning of the tape). Within each group, four minor numbers are available to define devices with different characteristics (block size, compression, density, etc.) When the system starts up, only the first device is available. The other three are activated when the default characteristics are defined (see below). (By changing compile-time constants, it is possible to change the balance between the maximum number of tape drives and the number of minor numbers for each drive. The default allocation allows control of 32 tape drives. For instance, it is possible to control up to 64 tape drives with two minor numbers for different options.) Devices are typically created by: There is no corresponding block device. The driver uses an internal buffer that has to be large enough to hold at least one tape block. In kernels before 2.1.121, the buffer is allocated as one contiguous block. This limits the block size to the largest contiguous block of memory the kernel allocator can provide. The limit is currently 128 kB for the 32-bit architectures and 256 kB for the 64-bit architectures. In newer kernels the driver allocates the buffer in several parts if necessary. By default, the maximum number of parts is 16. This means that the maximum block size is very large (2 MB if allocation of 16 blocks of 128 kB succeeds). The driver's internal buffer size is determined by a compile-time constant which can be overridden with a kernel startup option. In addition to this, the driver tries to allocate a larger temporary buffer at run-time if necessary. However, run-time allocation of large contiguous blocks of memory may fail and it is advisable not to rely too much on dynamic buffer allocation with kernels older than 2.1.121 (this applies also to demand-loading the driver with kerneld or kmod). The driver does not specifically support any tape drive brand or model. After system start-up the tape device options are defined by the drive firmware. For example, if the drive firmware selects fixed block mode, the tape device uses fixed block mode. The options can be changed with explicit ioctl() calls and remain in effect when the device is closed and reopened. Setting the options affects both the auto-rewind and the non-rewind device. Different options can be specified for the different devices within the subgroup of four. The options take effect when the device is opened. For example, the system administrator can define one device that writes in fixed block mode with a certain block size, and one which writes in variable block mode (if the drive supports both modes). The driver supports tape partitions if they are supported by the drive. (Note that the tape partitions have nothing to do with disk partitions. A partitioned tape can be seen as several logical tapes within one medium.) Partition support has to be enabled with an ioctl. The tape location is preserved within each partition across partition changes. The partition used for subsequent tape operations is selected with an ioctl. The partition switch is executed together with the next tape operation in order to avoid unnecessary tape movement. The maximum number of partitions on a tape is defined by a compile-time constant (originally four). The driver contains an ioctl that can format a tape with either one or two partitions. Device /dev/tape is usually created as a hard or soft link to the default tape device on the system.

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 do 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. 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 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.

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

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 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, 13 Boolean driver options, the buffer write threshold, defaults for the block size and density, and timeouts (only in kernels >= 2.1). 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 the constants MT_ST_BOOLEANS, MT_ST_SETBOOLEANS, MT_ST_CLEARBOOLEANS, or MT_ST_DEFBOOLEANS logically 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 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:

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 ORed with the operation code.

With kernels 2.1.x and later, the timeout values can be set with the subcommand MT_ST_SET_TIMEOUT or'ed 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.

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 */
};

NÁVRATOVÁ HODNOTA

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().)

EFAULT

The command parameters point to memory not belonging to the calling process.

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().

EROFS

Open is attempted with O_WRONLY or O_RDWR when the tape in the drive is write-protected.

FILES

/dev/st* : the auto-rewind SCSI tape devices /dev/nst* : the non-rewind SCSI tape devices

AUTHOR

The driver has been written by Kai Mäkisara (Kai.Makisara@metla.fi) starting from a driver written by Dwayne Forsyth. Several other people have also contributed to the driver.

SOUVISEJÍCÍ

mt(1) The file README.st in the kernel sources contains the most recent information about the driver and its configuration possibilities.

NOTES

1. When exchanging data between systems, both systems have to agree on the physical tape block size. The parameters of a drive after startup are often not the ones most operating systems use with these devices. Most systems use drives in variable block mode if the drive supports that mode. This applies to most modern drives, including DATs, 8mm helical scan drives, DLTs, etc. It may be advisable use these drives in variable block mode also in Linux (i.e., use MTSETBLK or MTSETDEFBLK at system startup to set the mode), at least when exchanging data with foreign system. The drawback of this is that a fairly large tape block size has to be used to get acceptable data transfer rates on the SCSI bus. 2. Many programs (e.g., tar) allow the user to specify the blocking factor on command line. Note that this determines the physical block size on tape only in variable block mode. 3. In order to use SCSI tape drives, the basic SCSI driver, a SCSI-adapter driver and the SCSI tape driver must be either configured into the kernel or loaded as modules. If the SCSI-tape driver is not present, the drive is recognized but the tape support described in this page is not available. 4. The driver writes error messages to the console/log. The SENSE codes written into some messages are automatically translated to text if verbose SCSI messages are enabled in kernel configuration.

COPYRIGHT

Copyright © 1995 Robert K. Nichols. Copyright © 1999 Kai Mäkisara. 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.