Tape Store-ies

Tapes have been around since 1972 when the first QICs (Quarter Inch Cartridges) were introduced by 3M to store data from telecommunications and data acquisition applications. Large-capacity tape-medium still remains the best option for full-system, network-wide data backups. Ever since, other tape-based mediums have emerged and are differentiated mainly on the basis of the methodology adopted to record data. There are two ways in which this can be done, in a spiral, or ‘helical’ fashion wherein the data tracks are written at an angle with respect to the edge of the tape called the HST (Helical Scan Technology) and the LST (Linear Serpentine Technology) where multiple linear tracks are written parallel to the edge of the tape. In HST, a magnetic tape is partially wrapped around an angled, rapidly rotating drum containing the read and write heads. In LST, the tape moves linearly over the head assembly where the precisely aligned read and write heads are located. 

Linear scan recording has stationary read-write heads. Data is written on tracks parallel to the tape’s edge

The 4mm DAT (Digital Audio Tape), Exabyte’s 8mm Mammoth and Sony’s AIT (Advanced Intelligent Tape) are based on HST while Quantum’s Digital Linear Tape (DLT and SuperDLT) and LTO (Linear Tape Open) by IBM, Seagate and HP employ the use of

LST. 

What’s DAT?



4mm DAT (Digital Audio Tape) is used for storing CD-quality audio format. Digital Data Storage or DDS is a DAT-based computer-data storage format, developed by Sony and HP and released in 1989. So what is used with computers is DDS, not DAT, but the name DAT-drives has stuck. The latest DDS specification is DDS-4, which supports a data-transfer rate of 2.4 to 4.8 MBps and stores 20GB (without compression) to 40 GB (with compression) data. Like the previous DDS standards, DDS-4 is backward-compatible with the previous two generations of DDS products. 

The 8mm 



The 8mm tape is similar to DAT but with higher storage capacities. Two protocols differentiated by compression algorithms and drive technologies exist in the 8mm space–the Mammoth by Exabyte and the AIT (Advanced Intelligent Tape) by Sony and Seagate. 

Helical scan technology uses a rapidly rotating drum containing the read-write heads, around which the magnetic tape is wound

Exabyte introduced Mammoth in the year 1986. Mammoth drives used a better design than 8mm drives using 40 percent fewer parts and improved reliability by reducing tape wear and tension variation. This was achieved by what Exabyte call their TapeSafe technology which uses a dual-reel, direct-drive system requiring no capstan. This method controls tape movement precisely by monitoring the amount of tape on each reel. As the amount increases on one reel and decreases on the other, the system continuously adjusts the speed of the reels to maintain constant tape speed and tension. The latest Mammoth drive, Mammoth-2 (M2) delivers a 30 MB per second compressed transfer rate, and a capacity of up to 150 GB (60 GB native) with a SCSI interface. 

Sony’s Advanced Intelligent Tape (AIT) arrived in 1996, based on the 8mm Helican scan, but a different recording format that renders it incompatible with 8mm drives from Exabyte. The AIT format uses Sony’s Memory-In-Cassette (MIC) drive interface system, which consists of a 64Kbit memory chip built into the data-cartridge. This chip stores data pertaining to the tape’s system log, search map and other user-definable information. Users can access data on the tape immediately no matter what section of the tape is being accessed. This reduces the average time to access data to fewer than 20 secs on

AIT-drives.

AIT-1 drives originally offered a native capacity of 25GB. This was increased in 1999 to 35GB/90GB native/compressed, and by early 2001 performance had been improved to 4MBps and 10MBps respectively. Before that, AIT-2 had been introduced, fully read and write backward-compatible with AIT-1 and providing 50GB of native capacity per tape (100GB with compression) and a 6MBps sustained native transfer rate (12 MBps with compression). By 2001 compressed capacity had been increased to 13GB, with an associated transfer rate of 15.6MBps.

Starting with a native capacity of 25GB (compressed 90GB) and a Data Transfer Rate(DTR) of 4MBps (and 10MBps), in AIT-1, Sony’s AIT-3 drives now offer 100GB (260GB compressed) capacity with a DTR of 12MBps (and 31.2MBps).

Digital Linear Tape 



Digital Linear Tape technology was developed by DEC (Digital Equipment Corporation) to be used with its MicroVax systems. Quantum acquired the technology from DEC in 1994 and has since then has been selling various DLT formats.

DLT-drives have a unique HGA (head-guide assembly), which minimizes tape wear. The HGA is a boomerang-shaped aluminum plate with six large bearing-mounted rollers. While in helical scan systems, tape is grabbed from the middle and pulled into place, the DLT HGA system links a leader strip on the end of the tape, pulls the tape out of the cartridge and wraps it around the take-up reel, guided the rollers. 

Quantum introduced the Super-DLT range in 1998, with a much higher capacity (110 GB native in SDLT 220 compared with 40 GB native in DLT8000). The latest SDLT drive from Quantum offers 160 GB native and 320 GB compressed capacity, with 16 MB/sec native and 32 MB/sec compressed transfer rate. 

The main advantages of DLT are the higher storage capacity, higher data transfer rates, and higher reliability because of the HGA which does not allow the media to touch the head in the drive. 

LTO



Linear Tape Open is the ‘open’ standard endorsed by HP, IBM and Seagate. LTO offers the Ultrium tape format. Ultrium format Generation 1 allows up to 100GB capacity (uncompressed) in a single cartridge, while Ultrium format Generation 2 doubles the capacity up to 200 GB (uncompressed) using Generation 2 data cartridges. The data transfer rate is upto 20MBps native in the Ultrium format Generation 1 and 40 MBps native in Generation 2. Ultrium Generation 2 was launched in April 2002. 

The technological advantages of LTO technology include linear multi-channel, bi-directional formats, data compression, track layout and error correction code. The four-generation road-map of LTO technologies claims doubling capacity and data transfer rates with every generation. LTO-based Accelis format was introduced in the year 1997 and was meant for applications that required fast access time. It has now been discontinued by

LTO. 

Being an open standards-based technology, LTO Intellectual Property licenses are available for sale, which allows any manufacturer to use, study and build on the formats offered by LTO. IBM was the first to ship Ultrium 1 drives in September 2000. HP first offered Ultrium 2 drives in November 2002.

Shruti Pareek