The DLT Digital Linear Tape drive
The DLT Digital Linear Tape was introduced in the mid-1980s. DEC Digital Equipment Corporation developed a new technology, which was based on standard ½ inch magnetic tape. The DLT was used with its highly successful MicroVAX system.
The DLT system was introduced in 1989 and later in 1994, the technology was acquired by Quantum Corporation. DLT is based on the old method of reel to reel magnetic recording, where the tape cartridge performs as one reel and the tape drive as the other.
The DLT drives use a computer controlled dual motor system. This dual motor system precisely controls the tape acceleration, deceleration and also the write/read speed. The DLT drive is designed for self-cleaning. For the cleaning purpose, there are two non-energised islands in the drive that continuously clean the DLT tape. They ensure that the tape and head are properly in 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.
The prime advantages DLT retains are higher storage capacity, higher data transfer rates, and higher reliability. The DLT provides beat performance mainly because the media does not physically touch the head in the drive. The DLT drive uses multiple channels simultaneously for data recording and reading. The DLT technology segments tape media into horizontal, parallel tracks and data is recorded 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 DLT drives have high transfer rate speeds both for backup and reading of large blocks of data. The 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 optimizes performance over a wide range of host data rates. DLT incorporates 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 minimizing delays due to repositioning.
Another criteria for 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 uses a file mark index located at the logical end of the tape, which minimizes search time. With the help of this index, the DLT drive lists the tape segment address of each file on the tape. The DLT 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 through a multi-layer approach, provide data integrity. An ASIC application-specific integrated circuit 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.
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About the Author: ShawnPaul
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