HP DDS/DAT tape drives UNIX, Linux and OpenVMS configuration guide (DW049-90930, May 2010)

DDS-4 uses longer tapes (150m). HP's DDS-4 drives, which are ultra-wide SCSI
devices, allow transfer rates from 3 to 4 times greater than DDS-3 and capacities
that are two-thirds as much again.
DAT 72 tapes are 170m long and data is written in narrower tracks, again
increasing data density, enabling tapes to hold 80% more data than DDS-4
tapes.
DAT 160 tapes use a wider tape, which allows 120% more information to be
stored than on DAT 72 tapes.
DAT 320 tapes use the same wider tape as DAT 160 but use a different coating
technology to provide increased data densities, which allows 100% more data
to be stored than on DAT 160 tapes.
filemark A mark written by the host to the tape that 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.
group A fixed capacity set of tracks written to or read from tape, defined in the DDS
format.
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. For writing filemarks, Immediate mode can significantly improve
the performance of systems that 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, by which different devices at a particular SCSI ID can be
addressed individually. The drive has a fixed LUN of 0.
Media Recognition
System (MRS)
A method by which a drive can recognize data-grade tape. The tape has a series
of stripes on its transparent leader tape that the drive can detect. By default, the
drive treats a non-Media Recognition System tape as read-only and will not write
data to it.
partition A part of a tape that can be treated as a complete and independent whole. A
tape can have one or two partitions.
SCSI Small Computer System Interface
sequential access Sequential access devices store data sequentially in the order in which it is
received. Tape devices are the most common sequential access devices. Devices
such as disk drives are direct access devices, where data is stored in blocks, not
necessarily sequentially. Direct access allows for speed of retrieval, but is
significantly more costly.
Glossary46