UNIX Configuration Guide:
Glossary

AT&T mode

Berkeley and AT&T functional modes differ in "read only" close functionality. In AT&T mode, a device close operation will cause the tape to be repositioned just after next tape filemark (the start of the next file).

Berkeley mode

Berkeley and AT&T functional modes differ in "read only" close functionality. In Berkeley mode the tape position will remain unchanged by a device close operation.

Compression

A procedure in which data is transformed by the removal of redundant information to reduce the number of bits required to represent the data. This is done by substituting short codewords for strings of bytes.

Filemark

A mark written by the host to the tape, which can be searched for, often using the drive's fast-search capability. It does not necessarily separate files. It is up to the host to assign a meaning to the mark.

Immediate mode

A mode of responding to SCSI commands where the drive or other peripheral does not wait until the command has finished before returning status information back to the host. When writing filemarks, Immediate mode can significantly improve the performance of systems which do not set the Immediate bit when sending a SCSI WRITE FILEMARKS command. On the other hand, data is not flushed to tape in response to a filemark command.

Infinite flush

By default, the buffer in the drive is flushed every 5 seconds. Infinite flush avoids frequent starting and stopping of the mechanism when using a very slow application. It also avoids losing capacity through the flushing of partly written groups. On the other hand, infinite flush means that data can remain in the buffer for very long periods of time, and could be lost in the event of a power failure.

LUN

Logical Unit Number. A unique number by which a device is identified on the SCSI bus. A tape drive has a fixed LUN of 0. In an autoloader, the changer mechanism is LUN1.