ct(7)

DESCRIPTION

This entry describes the actions of the general HP-UX Command-Set 1980 (CS/80) cartridge tape drivers when referring to a CS/80 cartridge tape as either a block- or character-special (raw) device.

Cartridge tapes are designed to work optimally as "streaming" devices, and are not designed to start and stop frequently. Technically, they are "random access" devices such as disks, but such access is both less efficient and more stressful than streaming mode. Thus it is possible to use a cartridge tape as a file system, or in general access it randomly, but such use will more rapidly wear either or both the tape drive and the media.

Cartridge tape units in either CS/80 disk drives or in stand-alone devices can be accessed as blocked or raw devices.

Block special files access cartridge tapes via the system's normal buffering mechanism. Buffering is done in such a way that concurrent access through multiple opens or a mount of the same physical device do not get out of phase. Block special files may be read and written without regard to physical cartridge tape records. Each I/O operation results in one or more logical block transactions. In general, this mode is not recommended as it stresses the hardware.

There is also a raw interface via a character special file which provides for direct transmission between the cartridge tape and the user's read or write buffer. A single read or write operation results in exactly one transaction. Therefore raw I/O is considerably more efficient when many bytes are transmitted in a single operation because blocked cartridge tape access requires potentially several transactions and does not transmit directly to user space.

In raw I/O, there may be implementation dependent restrictions on the alignment of the user buffer in memory and its maximum size. Also, each transfer must occur on a record boundary, and must read a whole number of records. The record size is a hardware-dependent value.

Selecting the proper buffer size when accessing a cartridge tape device through the raw interface is critical to the performance of the cartridge tape device and other devices connected on the same HPIB. A large buffer in certain situations can increase performance but has the potential to block other devices on the HPIB until all the data for a request has been transferred. On the other hand when a small buffer is used and the application is unable to keep the cartridge tape device streaming, performance and the wear and tear of the device suffer because of tape repositioning. The optimal solution is to keep the tape streaming while using a small buffer. To select the proper buffer size, consider two factors: the cartridge tape device being accessed and the application which is accessing the cartridge tape device.

Some cartridge tape units (see DEPENDENCIES) support a feature called immediate report mode. During writing, this mode enables the drive to complete a write transaction with the host before the data has actually been written to the tape from the drive's buffer. This allows the host to start gathering data for the next write request while the data for the previous request is still in the process of being written. During reading, this mode enables the drive to read ahead after completing a host read request. This allows the drive to gather data for future read requests while the host is still processing data from the previous read request. When data is requested or supplied at a sufficient rate, immediate report mode allows the drive to stream the tape continuously across multiple read/write requests, as opposed to having to reposition the tape between each read/write request. Repositioning adds to the wear and tear of the cartridge tape device and decreases the performance. Some cartridge tape devices (see DEPENDENCIES) do not support immediate report mode and as such cannot stream across multiple requests.

If the cartridge tape device being accessed supports immediate report mode and the application can maintain a data rate that allows the cartridge tape device to stream multiple requests, a small buffer (1 Kbyte to 12 Kbytes) is suggested so that the HP-IB is not blocked for a significant amount of time. For cartridge tape devices that do not support immediate report mode or applications that cannot maintain a data rate that allows the cartridge tape device to stream multiple requests, a large buffer (64 Kbytes) is suggested so that the number of tape repositions is reduced.

Each raw access is independent of other raw accesses and of block accesses to the same physical device. Thus, transfers are not guaranteed to occur in any particular order. Having multiple programs access the cartridge tape is, in effect, random access, and is subject to the warnings above.

In raw I/O, each operation is completed to the device before the call returns. For block-mode writes, the data may be cached until it is convenient for the system to write it. In addition, block-mode reads potentially do a one (or more) block read-ahead. The interaction of block-mode and raw access to the same cartridge tape is not specified, and in general is unpredictable. Because block-mode writes can be delayed, it is possible for a program to generate requests much more rapidly than the drive can actually process them. Flushing a large number of requests could take several minutes, and during that time the system will not have use of the buffers taken by these requests, and thus will suffer a possibly severe performance degradation. If the tape is integral with the system disk, very little disk activity may be possible until the buffers are flushed.

Cartridge tape device file names are in the following format:

where the first r indicates a raw interface to the cartridge tape, the second r is reserved to indicate that this cartridge tape is on a remote system, the c# indicates the controller number, the d# optionally indicates the drive, and the s# optionally indicates a section number. The assignment of controller, drive, and section numbers is described in the system administrator's manual for your system.

WARNINGS

Like disks, the cartridge tape units in CS/80 disk drives can be accessed as blocked or raw devices. However, using a cartridge tape as a file system severely limits the life expectancy of the tape drive. Tapes should be used only for system back-up and other needs where data must be stored on tape for transport or other purposes.

ct does not support access of DDS and QIC cartridge tape devices.

DEPENDENCIES

AUTHOR

ct was developed by HP and AT&T.

SEE ALSO

mknod(1M), tcio(1), disk(7), intro(7), mt(7).