DLT

The origins of Digital Linear Tape date back to the mid-1980s when Digital Equipment Corporation (DEC) developed a new technology, based on standard half-inch magnetic tape, for use with its highly successful MicroVAX system. The first true DLT system emerged in 1989 and the technology was later acquired by Quantum Corporation in 1994. A number of OEMs have subsequently licensed the technology primarily for the purpose of manufacturing automated tape libraries. Effectively, DLT is an adaptation of the old reel to reel magnetic recording method where the tape cartridge performs as one reel and the tape drive as the other.

DLT drives use half-inch wide metal particle tapes - 60% wider than 8mm tape and the widest tape available - onto which data is recorded in a serpentine pattern on parallel tracks grouped into pairs. Each data track goes the entire length of the tape. When data is recorded, the first set of tracks is recorded on the whole length of the tape. When the end of the tape is reached, the heads are repositioned to record a new set of tracks, and the tape is again recorded on its whole length, this time in the opposite direction. The process continues, back and forth, until the tape is full. Current drives record either 128 or 208 tracks. The higher density is achieved by angling the data pattern on adjacent tracks using a technique called Symmetric Phase Recording (SPR). SPR's herringbone pattern eliminates the need for guard bands, thereby allowing greater track density.

What makes a DLT drive unique is the design of the head guide assembly (HGA). The DLT HGA is a patented boomerang-shaped aluminium plate with six large bearing-mounted rollers. Instead of grabbing the tape in the middle and pulling it into place, as with helical scan systems, the DLT guide system links a leader strip on the end of the tape and pulls it out of the cartridge, guides it around the head guide assembly in a smooth arc, and gently wraps it around the take-up reel in the drive. The rollers guide, but do not pull the tape. The wrap angles around the guide rollers are gentle and contact between tape and guide is minimised. The recorded side of the tape never touches the guides, minimising tape wear.

The drives use a computer-controlled dual-motor system that precisely controls tape acceleration, deceleration and write/read speed. Their design is also self-cleaning. Like tiny stationary squeegees, there are two non-energised islands that continuously clean the DLT tape, ensuring proper tape/head contact, and data integrity. All this results in a head life specified at 30,000 hours - compared to 2,000 hours for 8 mm helical scan devices.

Data is recorded and read using multiple channels simultaneously. DLT technology segments tape media into parallel, horizontal tracks and records data by running the tape past a stationary head. Current products record two channels simultaneously using two read/write elements in the head, effectively doubling the transfer rate possible at a given drive speed and recording density. The high transfer rate speeds both backup and the reading of large blocks of data. In the future, higher bit density will be achieved through the use of new read/write heads such as thin-film and magnetoresistive (MR) heads, boosting both capacity and transfer rates. In addition, higher data transfer rates will be delivered by increasing the number of channels recorded simultaneously.

Although data transfer rate is pivotal to drive performance, the fastest tape drive doesn't always deliver the highest throughput. Overall tape throughput depends greatly on the ability of the tape drive to track the data rate of the host system. If the transfer rate of a tape drive is faster than the host data rate, the tape must stop and reposition frequently, thus degrading performance. DLT technology optimises performance over a wide range of host data rates by incorporating a highly effective adaptive cache buffering feature to achieve maximum throughput. This feature improves throughput by monitoring the host system and dynamically adjusting cache buffering operations to match the host data rate, thus minimising delays due to repositioning.

Another measure of tape performance is the time required to locate a file. This criteria is especially important in near-line applications, such as image manipulation, that frequently search for files and append or restore data. DLT technology minimises search time through a file mark index located at the logical end of the tape. Using this index, which lists the tape segment address of each file on the tape, the drive "steps" to the track containing the file and performs a high-speed streaming search to the file. This feature enables DLT products to find any file in a 20-gigabyte capacity tape in an average of 45 seconds.

DLT technology provides this exceptional data integrity through a multi-layer approach. An application-specific integrated circuit (ASIC) chip interleaves a Reed Solomon error-correction code (ECC) of 16KB with every 64KB of user data, a 64-bit cyclic redundancy code (CRC), and a 16-bit error-detection code (EDC) for each 4KB of data, along with an overlapped 16-bit CRC on each user record. As an added safety measure, DLT products verify data by performing a read after each write command, and will automatically re-record data further down the tape if a recording error is detected.

The prime advantages DLT retains are higher storage capacity, higher data transfer rates, and higher reliability, mainly because the media does not physically touch the head in the drive. The following chart lists the principal performance characteristics of the various DLT standards prior to Quantum's announcement of a major new step in DLTtape technology - Super DLTtape - in 1998.

Standard	Capacity (n/c)	Interface	Maximum DTR
DLT2000	15/30GB	SCSI	2.5MBps
DLT4000	20/40GB	SCSI	3MBps
DLT7000	35/70GB	SCSI	20MBps