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History (1992): First Tape Array

By Data General, 8 to 20GB, starting at $30,000

Data General has developed the first tape array, a fault-tolerant subsystem including several DAT drives that can read and write tapes in parallel, which means a RAID technology.

It will be integrated in a subsystem of disk and tape arrays.

The fault-tolerant RAID concept was until now applied to disk arrays, not to magnetic tapes. Gigatape (Puchheim, Germany) had once developed a dual drive subsystem with 2 DAT units with parallel data writing, which doubles the transfer rate, but is equivalent to a RAID-1 without fault-tolerant capability.

The Clariion Tape Array launched by Data General Corp. (Westborough, MA) is a real RAID-3 according to the Berkeley University’s classification, with parallel writing of data blocks on 4 different tapes, plus a 5th one for parity. Transfer rate is multiplied by 4, just like the subsystem’s total on-line capacity. But here, there is no gain in the average access time that is the same as for 1 drive.

The subsystem is fault-tolerant since it is possible to reconstruct the data of any one of the five tapes if one is useless or lost.

There still is a major drawback that doesn’t happen on disk arrays. When the 5 tapes are full, you have to arrange them in order to afterwards introduce all of them in each one of the 5 drives to recover the data. You can only loose one.

And we could not find out if it was possible to put each tape in any sequence to recover data, but this isn’t a main problem since the label on each tape can also include a sequence number.

Data General has to nevertheless offer a special rack to store the 5x4mm DAT cartridges. Since it uses 3.5-inch 4mm DAT drives with a 90-meter 2GB tape that can reach up to 5GB with data compression (DG tables on a 2.5:1 compression ratio), a total capacity of 8 to 20GB can be reached on a desktop tower cabinet measuring 63.5cm high, 22.9cm wide and 63.5cm deep. As each drive has a 183KB/s transfer rate, the subsystem allows to write at 740KB/s and up to 2MB/s with data compression, which means 2.7 to 6.6GB/hour.

The integrated storage control processor includes a 20MB/s differential fast/wide SCSI-2 host interface, a single internal SCSI-2 channel to drives, and a host independent RAID type management firmware, all operated by a Motorola 68030 microprocessor at 25MHz with a 1MB DRAM and a 128KB ROM.

The stand-alone tape array starts at $30,000 and will ship in the spring of 1993, according to DG. Later, in the summer of 1993, the tape array will be integrated into DG’s new disk array to back it up.

This article is an abstract of news published on the former paper version of Computer Data Storage Newsletter on issue ≠57, published on October 1992.