  Digital Data Storage
Digital Data Storage (DDS) is a format for storing and backing up computer data on tape that evolved from the Digital Audio Tape (DAT) technology. In 1989, Sony and Hewlett Packard defined the DDS format for data storage using DAT tape cartridges.
Tapes conforming to the DDS format can be played by either DAT or DDS tape drives. However, DDS tape drives cannot play DAT tapes since they can't pick up the audio on the DAT tape.
DDS uses a 4-mm tape. A DDS tape drive uses helical scanning for recording, the same process used by a video recorder (VCR).
There are two read heads and two write heads. The read heads verify the data that has been written (recorded). If errors are present, the write heads rewrite the data. When restoring a backed-up file, the restoring software reads the directory of files located at the beginning of the tape, winds the tape to the location of the file, verifies the file, and writes the file onto the hard drive.
DDS cannot update a backed-up file in the same place it was originally recorded. In general, DDS requires special software for managing the storage and retrieval of data from DDS tape drives.
There are five types of DDS drives:
4mm Tape DDS1 (DDS-1): Stores 2gb native (4gb compressed) of data on a 90m cartridge.
4mm Tape DDS2 (DDS-2): Stores 4gb native (8gb compressed) of data on a 120m cartridge.
4mm Tape DDS3 (DDS-3): Stores 12gb native (24gb compressed) of data on a 125m cartridge. Uses PRML (Partial Response Maximum Likelihood). which eliminates electronic noise for a cleaner data recording. Native transfer speed: 1-1.2mb/sec; 2mb cache.
4mm Tape DDS4 (DDS-4): Stores 20gb native (40gb compressed) of data on a 150m cartridge. 4mm Tape DDS4 native transfer speed: 2.4-3mb/sec; 8-10mb cache.
4mm Tape DDS5/72 (DDS-5): Stores 36gb native (72gb compressed) of data on a 170m cartridge. 4mm Tape DDS5 native transfer speed: 3-3.5mb/sec; 8-16mb cache.
A 4mm Tape DDS cartridge needs to be retired after 2,000 passes or 100 full backups. You should clean your 4mm Tape DDS tape drive every 24 hours with a cleaning cartridge and discard the cleaning cartridge after 30 cleanings. 4mm Tape DDS tapes have an expected life of at least 10 years.
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