Content-type: text/html Man page of mtio

mtio

Section: Environments, Tables, and Troff Macros (7)
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NAME

mtio - magnetic tape interface  

DESCRIPTION

This description applies to all mass storage tape drives. The /dev directory special files rmt0{a,l,m,h},...,rmt31{a,l,m,h} refer to the mass storage tape drives. Tape drives can exist on different buses, and have different bus/formatter/controller dependencies.  

Special Files

For both the rewind and norewind special files, the unit number represents a symbolic count that has no connection with the actual plug or controller number of a particular tape drive. As each tape unit special file is created, the number counts up from 0 to 31 for a total of 32 tape drives.

The rmt0l,...,rmt31l special files are low density, the rmt0m,...,rmt31m special files are medium density (also known as triple density), and the rmt0h,...,rmt31h special files are high density. These special files cause a loaded and on-line tape to automatically rewind to the beginning-of-tape (BOT) when closed. Low, medium, and high density are relative to the densities supported on a particular tape drive.

For SCSI QIC tape devices, the rmt0a,...,rmt31a special files are QIC-24 density, the rmt0l,...,rmt31l special files are QIC-120 density, the rmt0m,...,rmt31m special files are QIC-150 density, and the rmt0h,...,rmt31h special files are QIC-320 density. See tz(7) for more information.

The nrmt0{a,l,m,h},...,nrmt31{a,l,m,h} special files do not cause a rewind when closed, regardless of density. When closed, the tape is positioned between two tapemarks. The norewind namespace for the QIC-24 example shown previously is nrmt0a,...,nrmt31a The rmt and nrmt special files are available to all operating system utilities that can perform I/O to tape.  

Magnetic Tape Operations

Magnetic tape ioctl system calls perform tape operations. The operations come under three ioctl request groups. The MTIOCTOP ioctl is used to issue tape operation commands. The MTIOCGET ioctl is used to obtain status. The MTIOCRDPOS ioctl is used to obtain tape position information from the tape drive.  

MTIOCTOP ioctl Commands

The mtop data structure is passed as a parameter to the MTIOCTOP ioctl. Please see the /usr/include/sys/mtio.h header file for a definition of the mtop structure.

The mt_op field specifies the MTIOCTOP tape command to be performed. The mt_count field specifies the number of times the command should be performed (where applicable).

The following MTIOCTOP tape commands are supported: Writes an end-of-file to the tape. Physically, an end of file consists of a tape mark. Repositions forward the number of files specified in the mt_count field. This command repositions the tape forward the specified number of tape marks. (Tape marks delimit files.) Upon successful completion of this command, the tape is physically positioned at the end of the specified number of tape marks. Repositions backward the number of files specified in the mt_count field. This command repositions the tape backward the specified number of tape marks. (Tape marks delimit files.) Upon successful completion of the command, the tape is physically positioned at the beginning of the specified number of tape marks.

Because MTFSF leaves the tape positioned at the end of a tape mark and MTBSF leaves it positioned at the beginning, these two commands are not strictly reciprocal operations. For example, if a tape is initially positioned at the beginning of tape (BOT) and the command MTFSF 1 is issued followed by the command MTBSF 1, the tape does not return to the BOT position. Instead, the tape is positioned on the BOT side of the first tape mark.

Repositions forward the number of records specified in the mt_count field. This command returns a failure if a tape mark is encountered to indicate that there were not as many records remaining in the file as there were records specified by the mt_count field. Repositions backward the number of records specified in the mt_count field. This command returns a failure if a tape mark is encountered to indicate that there were not as many records between the present position and the beginning of the file as specified in the mt_count field. Rewinds the tape. This command repositions to the beginning of the tape. Rewinds and unloads the tape. Does not perform any tape operation. Always returns success from a tape file. Enables the use of hardware-based write-back caching for better performance. Caching can speed tape transfer operations, thereby keeping the unit streaming more effectively. If you use the MTCACHE command, use the MTFLUSH command to flush cached data to media. SCSI tape drives that support caching have their cache turned on automatically by the driver; the MTCACHE command is unnecessary. Disables use of the controller's hardware-based write-back cache. Tape operations using this command are slower than tape operations using the MTCACHE command, but do not require use of the MTFLUSH command to guarantee that the data is immediately written to tape. Clears a serious exception. Certain operations cause the tape unit to go into a serious exception state. This can happen, for example, when the physical end-of-media foil is encountered. Typically, when a tape is in a serious exception state, all data transfer operations fail. Use the MTCSE command to acknowledge the exception condition and allow further operations to proceed. Clears a hardware or software error. Clears the subsystem. Enables end-of-tape detection. When the end-of-tape markers are reached, the tape is halted on the reel between the two end-of-tape markers. Only the superuser can issue this command. The MTENAEOT command remains in effect for the device until end-of-tape detection is disabled with the MTDISEOT command. This is the default mode after a system boot. Disables end-of-tape detection. When the end of tape is reached, the tape will run off the reel. Only the superuser can issue this command. The MTDISEOT command remains in effect for the device until end-of-tape detection is enabled with the MTENAEOT command. Flushes the hardware-based write-back cache. For tapes that have controller-based caching (for example, TMSCP tapes), use this command with the MTCACHE command. For tapes that have device-based caching (for example, SCSI tapes), use this command by itself. When caching has been enabled, writes to the tape receive a completion status when the data has been transferred to the cache, not when the data is transferred to the media. Use the MTFLUSH command to force a flush of the cache to physical media. Failure of this command with errno set to ENXIO means that the drive does not support the flush command. However, SCSI devices do not return ENXIO; therefore you cannot rely on the MTFLUSH command to determine whether caching is enabled. Failure with errno set to EIO indicates that the cache flush has failed. In this case, the application should retry writing records that have been written since the last successful MTFLUSH command. Retensions the tape by moving the tape one complete pass between EOT and BOT. Moves the tape to the end of recorded data. Erases the tape. Loads and rewinds the tape. Loads the tape. Unloads the tape. Enables scatter/gather IO for the readv() and writev() system calls. After this command, any readv() or writev() system calls will cause the tape driver to transfer all iovec buffers in the list in a single transfer to tape. Disables scatter/gather IO for the readv() and writev() system calls. After this command, each buffer provided in a readv() or writev() system call will be transferred by itself. Sends a SCSI LOCATE command to the tape drive, telling it to position the tape to the SCSI logical block address specified by the mt_count field. Sends a SCSI LOCATE command to the tape drive, telling it to position the tape to the device specific address specified by the mt_count field.

 

MTIOCGET ioctl Requests

The mtget data structure is passed as a parameter to the MTIOCGET ioctl. Please see the /usr/include/sys/mtio.h header file for a definition of the mtget structure.

The following list describes the fields of the mtget data structure: Provides driver-specific drive status information. This is the same information provided in the stat member of the devget structure used by the DEVIOCGET ioctl.

For the TMSCP driver, please see the /usr/include/sys/devio.h> header file for bit definitions of the stat member.
For the SCSI tape driver, please see the /usr/include/io/cam/cam_tape.h header file for bit definitions of the ts_flags member of the TAPE_SPECIFIC structure. Provides driver-specific error information.
For the TMSCP driver, please see table B-1 in the /usr/include/io/dec/sysap/mscp_msg.h header file for error code definitions.
For the SCSI tape driver, the mt_erreg member contains the sense key byte of the sense data from a SCSI REQUEST SENSE command. Please see the /usr/include/io/cam/scsi_all.h header file or the SCSI-2 standard for definitions of the sense key. Also included in this byte are the Filemark, EOM, and ILI bits as defined in the SCSI-2 standard. Contains command-specific results. For example, after a read command using a variable block-length tape, mt_resid contains the residual number of bytes not transferred. Another example is the space command. After a space command, mt_resid contains the number of blocks or files not spaced over. Contains the file position of the tape. Contains the record position within a file.

Extended error information can be found in the /usr/include/io/common/deveei.h header file.  

MTIOCRDPOS ioctl Requests

The mtrdpos data structure is passed as a parameter to the MTIOCRDPOS ioctl. Please see the /usr/include/sys/mtio.h header file for a definition of the mtrdpos, mtrdposs, and mtrdposl structures.

The following two structure members of the mtrdpos structure are information provided by the application to control the format of the data returned by the tape drive. The un member of this structure contains data passed back to the application from the tape drive.

This is a single bit which, if set, tells the SCSI tape driver to send the READ POSITION command, requesting that the tape drive return the long data format. This is a single bit which, if set, tells the SCSI tape driver to send the READ POSITION command, requesting that the tape drive return the short data format with device specific addresses, rather than the SCSI logical block addresses. If the long_format field is set, then this bit is ignored by the tape driver. This is a union of the short (mtrdposs) and the long (mtrdposl) data format structures. The values in the members of these data structures are copies of the information in the short and long data formats returned by the SCSI READ POSITION command as specified in the SCSI standard documentation. Please note that the first_blk member of the mtrdposs structure and the blk_num member of the mtrdposl structure return information in the form which may be used with the MTSEEK and MTSEEKDS commands of the MTIOCTOP ioctl.  

System Call Behavior

Each read() or write() system call reads or writes the next record on the tape. In the case of a write() system call, the record has the same length as the buffer given. During a read() system call, the record size is passed back as the number of bytes read, provided it is no greater than the buffer size. If the record is long, an error is returned. Seeks are ignored. Positioning is done with a tape ioctl call. A zero byte count is returned when a tape mark is read, but another read fetches the first record of the next tape file. When a file open for writing is closed and the last user command was a write, two end-of-file marks (EOF) are written. If a tape reaches the end-of-tape (EOT) marker, the ENOSPC errno value is set.

Each read() or write() system call causes the file offset associated with the device special file to be incremented. This file offset is reset to 0 when the file is closed. If a program does an unusually large number or reads and writes to the tape, it is possible to cause the file offset to be incremented beyond the maximum allowable value. When this happens, any further read() or write() system calls fail with an error number of EINVAL. This situation can occur only if the tape is read or written to several times over, using repositioning commands such as MTREW to reposition backwards on the tape. It is recommended that any application which expects to make numerous passes over the tape use the lseek() system call to reset the file offset to zero, for example, lseek(fd,0,0).  

RESTRICTIONS

The MTRETEN ioctl is supported only by the SCSI QIC tape drive.  

RETURN VALUES

For the DEVIOCGET and MTIOCGET ioctls, the DEV_TPMARK, DEV_SHRTREC, DEV_EOM, DEV_OFFLINE, DEV_SOFTERR, and DEV_HARDERR tape driver flags are in an indeterminate state unless the application has gotten an unexpected return value from a system call (that is, read x bytes from tape, and if the return value does not equal x, then the flags are not in an indeterminate state).

The MTIOCGET ioctl call is non-intrusive. This means that the device driver implements support for MTIOCGET solely by interrogating its data structures. The device should not be perturbed. This also means that the MTIOCGET ioctl call always returns ESUCCESS. However, because the implementation of the MTIOCGET ioctl is dependent upon each device driver, and upon each device driver's ioctl code, an errno status is sometimes returned. An errno status returned from a MTIOCGET call indicates an error condition inside the driver itself. These are usually pre-processing errors inside the device driver's ioctl code.  

ERRORS

The MTIOCTOP ioctl commands set errno to [ENXIO] if the command specified in mt_op is not recognized or not supported by the respective tape driver.

The following list describes the possible errno status returned from MTIOCGET: The operation was successful. An error occurred while trying to copy out ioctl data (FOP_IOCTL). The device database structure was not present (no nexus). The device-specific data structure was not present (device not opened). The device driver data structure was not present (device not opened). The request was invalid or the requested function is not supported. The requested function is not supported.  

FILES

/dev/rmt???
/dev/nrmt???  

RELATED INFORMATION

lseek(2), SCSI(7), tz(7), MAKEDEV(8) delim off


 

Index

NAME
DESCRIPTION
Special Files
Magnetic Tape Operations
MTIOCTOP ioctl Commands
MTIOCGET ioctl Requests
MTIOCRDPOS ioctl Requests
System Call Behavior
RESTRICTIONS
RETURN VALUES
ERRORS
FILES
RELATED INFORMATION

This document was created by man2html, using the manual pages.
Time: 02:40:19 GMT, October 02, 2010