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. DAT was created for CD-quality audio recording. 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 four types of DDS drives:
- DDS-1: Stores up to 2 gigabytes of uncompressed data on a 120-minute cartridge.
- DDS-2: Stores up to 8 GB of data in compressed format on a 120-minute cartridge. DDS-2 is ideal for small network servers.
- DDS-3: Stores up to 24 GB of data on a 125-minute cartridge. The DDS-3 drive is ideal for medium-sized servers. DDS-3 uses PRML (Partial Response Maximum Likelihood). PRML eliminates electronic noise for a cleaner data recording.
- DDS-4: The newest DDS drive, DDS-4 stores up to 40 GB of data on a 125-minute cartridge. Small to mid-size businesses benefit from the DDS-4 drive.
A DDS cartridge needs to be retired after 2,000 passes or 100 full backups. You should clean your DDS tape drive every 24 hours with a cleaning cartridge and discard the cleaning cartridge after 30 cleanings. DDS tapes have an expected life of at least 10 years.