DDS-2 tape autoloader: high-capacity data storage in a 5.25-inch form factor - HP C1553A DDS-2 DAT tape autoloader - includes related articles on the autoloader's control electronics and firmware design

Hewlett-Packard Journal, Dec, 1994 by Steven A. Dimond

The autoloader holds six 4-gigabyte cartridges. With data compression, it can back up typically 48 Gbytes of data overnight or 8 Gbytes every day for six days, unattended.

The trend from centralized computing data centers to PCs and client-server networks has led to increased local or semilocal data storage and backup. As discussed in the preceding article, DDS tape drives are designed to meet these requirements. However, as network server disk capacities increase above the capacity of a single DDS tape (8 gigabytes for DDS-2, compressed), or if manipulation of the backup tapes becomes a chore, then there is a requirement for a larger storage device.

The type of person who carries out the backup has also changed with these trends in computing. The centralized data center had trained, full-time operators, whereas the network administrator or workstation user may not be formally trained and will want to spend the minimum time and effort completing the backup.

These requirements for storage capacity and ease of use have led to a need for an automated, easy-to-use, large-capacity tape device.

One way to add significant capacity at modest cost is to use a changer mechanism (robot) to select a tape from a library of tapes and put it into the tape drive. The changer mechanism may only double or quadruple the cost of the tape drive unit, but the capacity can increase many times more than this. There are tape libraries that have from 10 to 120 tapes and one or two built-in tape drives. The access times to select a tape are acceptable for a backup or library type application.

Given the emerging network requirements, there is an even bigger need for a small device that fits the standard "5 1/4-inch" peripheral slots. These are approximately 146 mm wide by 83 mm high by 203 mm deep (5.75 in by 3.25 in by 8 in). This is enough volume to hold a smaller peripheral-size tape drive and a changer mechanism.

These smaller devices that perform unattended backup are typically called autoloaders. At HP's Computer Peripherals Bristol division, we investigated this growing need. This investigation led to the development of the HP C1553A DDS-2 digital audio tape autoloader, Fig. 1.

The HP C1553A autoloader incorporates the HP C 1533A DDS-2 tape drive described in the article on page 6. It holds six DDS-2 cartridges, each having a native capacity of four gigabytes. With data compression, each cartridge can hold typically eight gigabytes, giving the autoloader the ability to back up typically 48 gigabytes without operator intervention. Two standalone versions are available: The HP 6400 Model 48AL for HP 9000 workstations and the HP SureStore-Tape 1200e for Novell Netware and Windows NT systems.

Design Objectives

The basic definition for the HP C1553A autoloader was very simple. It was to be a DDS tape autoloader, fit into a standard 5 1/4-inch peripheral enclosure, use a standard HP DDS drive with minimum (or no) modifications, use the drive's SCSI II interface, hold as many tapes as possible, and be reliable and ergonomic.

During the investigation phase for this product there were prototypes of similar products available. To keep the investment low we considered procuring one of these designs. However they fell short of some of the requirements, so the decision was made to produce our own design.

Given the small size of the product and the desire to accommodate as many tapes as possible, the interior space was at a premium. Despite frequent questioning by the engineers, the outside dimensions could not be increased if we were to be sure of satisfying the maximum number of customers. One possibility was to have a "power bulge" on the rear of the unit, but this was rejected because it might obstruct some customers' installations.

It was decided that the new HP C 1533A DDS-2 tape drive was to be fitted inside the autoloader. For reasons of manufacturing simplicity and cost, the drive had to be used with minimum modification. We were also aware that the autoloader would be a platform for future DDS drives, so easy integration was important for future generations as well. The price of the autoloader could perhaps be only double that of the DDS-2 drive for several times the capacity.

The SCSI II interface of the built-in drive can be used to pass on control commands to the changer mechanism. Thus the customer need only use one SCSI bus ID where some libraries require two.

The more tapes the device can hold, the more attractive a product it will be. Competing autoloaders with four cartridges were known to be under development, so our goal was to match this number or exceed it.

We saw the magazine holding the tapes as a simple storage solution, that is, inexpensive and capable of being stored like a video tape. This allows the user to treat a magazine as a big single backup tape, rather than having to manipulate a lot of single tapes.

For reliability and ease of use, use models and metrics were developed. In the early stages of development, user tests were conducted on possible design concepts.