Operating Systems

Tape continues to be the lowest- cost option for data storage and is an essential component of data backup and disaster recovery. Recent advances in tape technology provide even more compelling value—significantly decreasing costs and lowering access speeds. Virtual tape systems ensure the use of the full tape media, DASD caches provide access to tape data at DASD speeds, and robotic drives decrease mount speed when real mounts are required.

These technological and cost-saving advances have led to an increase in the number of applications that rely on tape. As tape usage increases, it’s essential to manage it with the same high-quality tools and solutions used to manage any other critical resource.

A Brief History of Tape

In the ’50s, magnetic tape became the first widely used data storage medium. Tape reigned as the dominant medium until the emergence of DASD. The faster data access speeds of DASD improved the performance of legacy applications and enabled the development and deployment of today’s transaction- oriented applications. Tape data density, reliability, and speed of access continuously improved, but three fundamental problems—inefficient media storage, speed of data access and tape drive limitations—relegated tape to handling less mission-critical roles such as data backup and late-night, batch-oriented applications.

Inefficient media usage was the biggest problem. Studies in the ’90s found that the average tape cartridge at a typical corporate data center was only about 10 percent utilized. The fact that about 90 percent of the total tape storage capacity was unused and unavailable meant that the incredibly low cost-per-byte of storage for tape, which was calculated based on full utilization, was wrong. In the real world, tape storage was much more expensive than in the theoretical world, where the assumption was that every cartridge was filled to capacity.

The second problem was speed of data access. Even with the introduction of robotic drives, it still took 20 to 30 seconds to mount a specific cartridge and additional time to position the tape to retrieve specific data. When contrasted with the millisecond data access times for DASD, tape clearly couldn’t fill a robust role in many strategic applications.

Finally, unlike DASD devices that can be simultaneously accessed by multiple applications, a tape drive can be used by just one application at a time. Since tape drives are expensive, most data centers couldn’t justify purchasing the number of tape drives required to satisfy workload requirements during peak production periods. This meant that many applications designed to use tape couldn’t take advantage of it; the only ones that could were those that could afford to wait their turn.

A Tape Renaissance

The introduction of virtual tape systems in the late ’90s solved all three problems and led to a resurgence of tape as the most cost-effective storage medium.

The DASD cache at the heart of a virtual tape system solves the huge problem of underutilized tape media. Virtual tape volumes are created, accessed, and updated in the DASD cache and are stacked on physical tape cartridges for permanent storage as a background activity. The physical cartridges are filled to 100 percent capacity. This ensures full use of the storage media and saves on storage slots.

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