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Tape Drive Reliability

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Reliability: How is it Measured?

Systems managers regularly see many conflicting claims of product reliability from competing hardware manufacturers, who cite mean-time-between-failure (MTBF) measurements or annualized-failure-rates (AFR) as the basis for their claims. MTBF quantifies the probability of failure for a product, however, when a product is first introduced: this rate is often a predicted number and only after a substantial amount of testing or extensive use in the field can a manufacturer provide demonstrated or actual MTBF measurements. Prior to introducing a product, Quantum performs extensive testing, under strenuous conditions, at both the product and component level to arrive at the specified MTBF. For example, stress tests for environmental limits assure operating temperatures of 50 to 104_F, non-condensing humidity of 20 to 80%, and altitude operation up to 40,000 feet. During a products life-cycle Quantum revises the MTBF based on actual field data.

AFR is similar to MTBF and differs only in units. While MTBF is the probable average number of service hours between failures, AFR is the probable percent of failures per year based on the manufacturer's total number of installed units of similar type. However, to be meaningful, both of these "standard" measures of reliability must be carefully scrutinized to identify the conditions under which they are achieved.

What Are the Operating Conditions?

Presently, some 8mm and other tape drive suppliers are claiming superior reliability ratings when compared to Quantum's DLTtape system. The key question that a systems manager should ask is: "Under what conditions are the claims and comparisons being made, how current is the data, and is the data based on the manufacturer's own qualified statistics?" For example, recently cited reliability figures comparing 8mm and other drives to the DLTtape system are not based on any empirical data, but are based on the findings of a survey by a company that admittedly hasn't provided tape reliability data since 1994. Additionally, the published figures are based on pre-1994 product models that are not being manufactured or sold today. Further, the reliability figures cited for 8mm and other drives are achieved only when their tape drives are operated at a duty-cycle of 10%, according to their published specifications. 10% duty cycle is not a realistic usage figure in today's backup environments.

In other words, these manufacturers achieve their given reliability ratings only when operating their products 10% of the time or less, then compare this rating to an Annual Failure Rate for older DLTtape systems derived from a 1994 survey rather than manufacturer's official empirical data.

Quantum designed the DLTtape system to operate at 100% duty cycle: reliability figures have not been quantified at duty cycles as low as 10%. Why not? Such a light usage reliability test is seldom relevant in the mission critical applications for which the DLTtape system was designed. Table 1 below shows MTBF figures for the DLTtape system and 8mm drives. Table 2 shows Quantum's actual DLTtape system field reliability data compared to 8mm and DAT tape drives.

Table 1. MTBF Based On Duty Cycle - DLTtape and 8mm

DLT7000*DLT4000*DLT2000XT*Sony 8mmExabyte 8mm

MTBF (hrs)

Duty Cycle100%100%100%10%20%

* DLTtape drive MTBF figures based on actual field data.

Table 2. Annual Failure Rate Based on Technology

DLTtape*8mm**4mm (DAT**)

* Average AFR for Quantum's DLTtape system based on actual field data.

    • Source: Strategic Research Corp.

      Operational Availability: The "Acid


Another key measure of reliability is Operation Availability (OA) which is the probability that a system will be fully functional and available at any given time. One of the most important determinants of OA is the product's reliability as measured under high duty-cycle conditions. Other tape drive manufacturer's claim that their products fail less often, but may base their comparisons on measurements made operating their products with duty cycles of only 10% thus assuming their products are not moving tape 90% of the time. Again, MTBF comparisons are meaningless in applications requiring high duty-cycles when reliability measurements aren't achieved in rigorous use. Reliability measures achieved under realistic, high-use conditions are the "acid test" that clearly identifies superior products.

One Challenging Application is Worth a Thousand Expert Opinions

Quantum's DLTtape system was designed to set a new standard of reliability in the industry, and has done so. While designed for non-stop, rigorous use in commercial environments, the DLTtape system has been taken beyond their design envelopes by many government and military agencies impressed with their reliability.

NASA Chooses the Quantum DLTtape System

In a truly "mission critical" application using arrays of tape drives, NASA stores streams of high-speed data with Quantum's DLTtape system, sometimes for months before launches as well as during the actual flights themselves. Data from the mission flows to banks of DLTtape drives to be stored on DLTtape cartridges available for use worldwide. For mission-critical applications data loss is simply not an option!

High Performance at High Altitude

In the jet aircraft related applications, Quantum's DLTtape system routinely survives Mach-2 test dives from 50,000 feet in F-15 Eagle jet fighters. Reliable operation even under the excessive G-forces and other conditions experienced in military aircraft is an example of Quantum's DLTtape system performing beyond expectations in challenging real-world environments. DLT4000 and DLT7000 tape drives were selected for use in the MARS II-SPEC system to record critical in-flight data during the testing of military aircraft, avionics, and weapons systems around the world. Test managers find that even in temperatures above 120 degrees (F), Quantum's DLTtape system mounted in special enclosures, perform at the specified data transfer rate without detectable change in error rates (1.5 MB per second for the DLT4000; 5MB per second for the DLT7000).

Nuclear Collisions Captured on the DLTtape System

Approximately once per second, an electron-positron "annihilation" occurs in a sophisticated accelerator buried 40 feet below Cornell University's athletic practice fields. Each nuclear collision generated by the University's half-mile circular particle racetrack, called the Cornell Electron Storage Ring (CESR), yields valuable research data, data that is captured and analyzed 24-hours a day, 7-days a week. Cornell states that the DLTtape system outlasts 8mm technology tape drives by a factor of six. Additionally, they found they were forced to clean the 8mm helical scan drive heads ten times more often than the manufacturer's guidelines specified, sometimes several times in one day. With the helical scan drives, they never knew whether a tape written by one drive would be readable by another, a problem eliminated by the DLTtape system. The DLTtape system at Cornell runs at 75% or greater duty cycle daily for a year and a half with no problems, according to Cornell.

A Whole New Paradigm

Avionics Engineering Division, one of the world's largest aerospace firms, relies on a Quantum DLTtape library system to perform daily incremental and weekly back-ups of vital engineering work - 120 gigabytes of data a week. The division's Computer Resource Center experienced frequent read errors with its previous DAT system, making it difficult to recover files and directories. The Center ruled out 8mm drives due to reliability problems being experienced by another group. "In terms of reliability, the DLTtape system has it, no question," says the CRC group leader.

One Picture is Worth a Megabyte

The worldwide leader in video-conferencing is PictureTel Corporation with more than 35,000 installed systems. PictureTel was experiencing a wide variety of problems and unacceptable failure rates with their installed 8mm tape drive libraries. One PictureTel manager stated, "With 8mm, we were constantly playing catch-up (replacing tape drives)."

Their remedy was to replace the failing 8mm products with Quantum DLT4000 tape drives. In addition to achieving an instant increase in reliability, PictureTel also found that the DLTtape drives were easier to maintain, "The DLTtape drive read-write heads had such a light touch we didn't have to clean the heads." With the cost of hard drives continuing to decline, PictureTel is expanding their systems' capacities by installing tape libraries in cost-effective increments of 10 to 16 tape slots that can be upgraded to 138 tapes, a system storage capacity of 4.9 terabytes, uncompressed. In-house, PictureTel backs up approximately 200 workstations on their network using DLT4000 tape drives.

The DLTtape System: Performance. Reliability. Scalability. Best In Class Technology

In the final analysis, industry counts on Quantum's leading and proven DLTtape system for backing up and retrieving mission-critical information. And because IT organizations have demanded the DLTtape system, all major computer and tape library vendors now offer it to meet their customers' needs. The storage professionals at Quantum help customers create high availability systems that perform reliably under nearly every condition in widely varying applications. The DLTtape system has earned it's superior reputation through customer satisfaction, and continue to lead the industry with an MTBF of over 200,000 hours at 100% duty cycle coupled with a demonstrated low AFR.

See Also