For almost as long as computers have existed, magnetic tape has been the backup medium of choice. Tape is inexpensive, well understood and easy to remove and replace.
But as hard drives grew larger and databases became massive data warehouses, tape had to change to store more data and do it faster. From 10-in. reels of 0.5-in. 9-track mainframe tape, focus shifted to the speed and convenience of tape cartridges.
Over the past 30 years, there have been many tape cartridge formats, and even today several formats are widely used. This QuickStudy aims to sort out differences among them and help explain which formats best suit various needs.
Linear Tape-Open (LTO)
This open-format technology was jointly developed by Hewlett-Packard Co., IBM Corp. and Seagate Removable Storage Solutions (now Certance LLC), beginning in 1997.
"Open" means that tapes and drives from different manufacturers are compatible with one another. Historically, tapes could often be read only by the drive that wrote them.
LTO uses linear, multichannel, serpentine (back-and-forth) recording on 0.5-in. tape with magnetic servo for error correction and hardware data compression. An embedded electronics module can store and retrieve usage and other information about a cartridge. LTO technology was originally announced in two variants, Accelis and Ultrium, aimed at speed and capacity, respectively. However, there was no demand for Accelis, and it has since been withdrawn.
Ultrium, a direct competitor to Super Digital Linear Tape (SDLT), uses a single tape spool inside a cartridge. The current second-generation Ultrium-2 tapes can store 200GB of data in native mode, or 400GB if compression is used. A March 2003 report by Gartner Inc. analyst Fara Yale states that more than twice as many Ultrium drives as SDLT drives were shipped in 2002.
Digital Linear Tape (DLT)
Developed by Digital Equipment Corp. in the 1980s, DLT is an adaptation of older reel-to-reel mainframe recording practice, where the removable media uses a single reel of tape and the drive handles the takeup end. According to the DLT Web site (www.dlttape.com), more than 2 million DLT drives have been sold, as well as over 90 million tape cartridges. In the most recent version, SDLT 320, optical lasers use servo tracks on the back of the tape to align the magnetic recording heads. Current SDLT drives can read older DLT media.
Advanced Intelligent Tape (AIT)
Developed in 1996 by Sony Corp., AIT uses helical-scan recording on 8mm tape, similar to that used in Hi-8 video camcorders. With a higher bit density and narrower tape, AIT cassettes are smaller than other tape cartridges, allowing for tape libraries that hold more data but take up less space.
AIT's small size makes it a good choice for organizations migrating from older systems based on the Digital Data Storage standard using digital audio tape. Sony is the only supplier of drives and media.
AIT cassettes include a memory chip inside the media cartridge to record and store format and file-location information. This lets AIT tapes load faster and cuts file search times in half.
AIT drives use IBM's Advanced Lossless Data Compression (ALDC) technology, which offers compression averaging 2.6:1 across multiple data types, vs. the 2:1 average for normal compression. Super AIT (SAIT-1) is essentially the same as AIT-3, but it uses 0.5-in. tape, giving it 500GB native capacity.
Introduced in 1996 by Exabyte Corp., Mammoth is aimed at the midrange server market.
The latest Mammoth-2 drive uses a multichannel helical scanning head on 8mm tape, error-correction and ALDC compression.
A unique technology developed by Ecrix (now part of Exabyte), VXA reads and writes data in packets. It operates at variable speeds, so it can match the data transfer rate of the host and, unlike drives that operate at fixed speeds, it doesn't have to stop and wait if the host is transferring data at a slower rate. That reduces wear on drives and media. VXA heads can read data from any physical location on the tape, without having to follow tracks from beginning to end.
Travan drives use a linear, single-channel recording on 0.25-in. tape. Its lower capacities and inexpensive hardware make it suitable for remote locations, small offices and individual workstation backups.