HP Q1581A HP DDS/DAT drives UNIX, Linux and OpenVMS configuration guide (DW049 - 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 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 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. 40