HP 157770-001 - DAT Drive 20/40 Tape Configuration Manual page 40

DDS Digital Data Storage is a recording format that builds on the DAT format to support the
storage of computer data. It was developed originally by Hewlett-Packard and Sony as
an industry standard. The first generation standard was DDS- 1 (or simply DDS), to
which was added data compression to produce the DDS-DC standard.
Further enhancements, notably narrower tracks and thinner tape, led to DDS-2, which
can typically provide double the capacity of DDS- 1 .
DDS-3 used a new magnetic coating on the tape that allows twice the recording
density. Together with the use of time-tracking, this gives a DDS-3 tape approximately
three times the capacity of a DDS-2 tape.
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.
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
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 A method by which a drive can recognize data-grade tape. The tape has a series of
System (MRS) 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.
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WRITE FILEMARKS