Helical Scan Vs. Linear Recording Tape Drives.

SUSCEPTIBILITY TO DUST

As the importance of vital business data residing in today's computer systems has become clear to the millions of professionals relying on the availability of their data, so has the importance of proper routines for protecting this data. Today, by far, the most common way of protecting mission critical data is to take backup copies of the data onto magnetic tape.

As a result of this steady increase in use of tape backup systems, two major tends become apparent:

1. Just like every other part of the computer business, the need for speed, capacity, and reliability for tape backup devices never stops growing.

2. Tape drives are required to operate in the same environments as other types of computer equipment, (from dedicated, clean, and well ventilated computer rooms of data centers to the work floor of garages, pizza bakeries, and stone quarries).

What sets the tape backup devices apart from other components in the computer systems is the fact that these devices must operate without the protection of the hermetic seal that protects the computer's hard disk, microprocessor, or other electronic components. Therefore, the tape backup system must operate and perform to specifications while being subjected to intrusion of dust, fumes, and other types of airborne contaminants: your tape drive's worst enemy.

Different Tape Backup Technologies--How To Choose

Today's tape backup devices ("drives") are being built around one of two technology platforms: linear recording or helical scan technology.

Helical Scan

The helical scan tape backup drives are derived from the technology used in video camcorders and DAT (Digital Audio Tape) recorders. Historically, helical scan drives have provided higher transfer rates than comparable linear tape drives, but at the higher cost of the complex and delicate design with lower reliability.

Linear Recording

Unlike those based on helical scan technology, tape drives based on linear recording technology have a much less complex construction with a less complex tape path and simplified mechanical design that provides robustness and reliability.

The Important Difference

Besides the obvious difference in mechanical complexity between linear recording and helical scan tape drives, there is one more important factor in regards to susceptibility to dust. This is the interface between the recording head in the drive and the magnetic media inside the cartridge.

A linear recording tape will pass over an edge that will actually remove the dust before reaching the head. A helical scan tape will gradually be wound around a rotating drum causing dust to be dragged in between the tape and the head. This will cause the dust to damage both the tape and the head and reduce reliability and performance. It should be noted that some helical scan tape drives (Sony AIT-1 and AIT-2, for example) have a built-in automatic head-cleaning device. This device is activated after the drive detects an operational problem and tries to correct the problem rather than prevent it.

Third-Party Tape Benchmark Dust Test

In the fall of 1999, Tandberg Data requested a third-party test house to perform a benchmark test of tape backup drives to assess their susceptibility to dust contamination. NTS*XXCAL in Los Angeles, CA was chosen as an independent third-party test house to perform the testing. NTS*XXCAL Computer Product Testing & Certification Labs, a division of National Technical Systems, was formed to serve the testing needs of microcomputer manufacturers in the areas of quality assurance, programming, compatibility testing and certification, functionality testing, usability testing, test plan development, and documentation.

Purpose

The main purpose of the benchmark was to compare helical scan and linear recording tape drives with respect to drive and media ability to withstand dust contamination. Based on the recognition of dust intrusion as a main contributor to potential problems with tape drives, the difference in tape path complexity and mechanical robustness, such a test was expected to prove the more inherent robustness of the linear recording technology.

The test was set up using standard PC hardware and software similar to that found in normal entry-level servers. The severity level (dust concentration and dust composition) in the test was chosen to simulate a normal office environment.

Models Tested

The test was performed using Tandberg Data's SLR (Scalable Linear Recording) tape drives, model SLR24 and SLR50, along with helical scan technology-based DDS drives from Hewlett Packard and Sony. Media for the SLR drives was supplied by Imation and for the DDS drives by Fuji Film (Table 1). Note: The Sony DDS4 drive used was an external enclosure model, i.e. the drive had extra protection from dust by being built into an enclosure with its own power supply.

Definition Of Serverities

Basically, dust tests consist of placing test devices inside a chamber and operating the devices while exposing them to some degree of dust. Various test standards call for either the dust to be blown into the chamber and circulated by airflow or to be released from the top of the chamber and allowed to settle freely onto the unit under test.

For this particular test, an international standard ([1] IEC standard 60068-2-68, method Lb) was chosen. Letting free-falling dust settle on the test devices without added (forced) airflow performs this test. This simulates the effect of dust accumulating on equipment located in an office or normal indoor work environment. By adjusting the amount of dust released into the chamber per 24-hour period, a test time of few days can simulate the effect of several years of exposure to dust sedimentation.

Ten Years Of Use

The amount of dust used in this test was set to 6g/m2 per 24 hours and the test duration to 144 hours (6 days). This is equivalent to 10 years' exposure to dust in a normal office environment.

Test Setup

To make a valid comparison between the various tape drives, NTS*XXCAL Test Engineers utilized the test bed as diagrammed in Fig 1. Each server platform was configured with its own unique tape drive (i.e., one server was configured with a Tandberg 5LR24 Tape Drive, the second server was configured with a HP DDS-3 Tape Drive, etc.). All tape drives were tested in their "shipped" configurations. In addition, each tape drive was tested under the same environment (e.g., same mounting location, etc.). Tests were performed on an isolated LAN to rule out the possibility of additional network traffic affecting the test results. Prior to the start of testing, NTS*XXCAL contacted each of the respective competitive vendors in turn, informing them that NTS*XXCAL was performing a benchmark/performance with their product while giving them a chance to submit the latest available released software and firmware revisions applicable to the benchmark/performance.

Runar Angelsen is the product manager of media at Tandberg Data ASA (Oslo, Norway).

                    Tape drives and media used in test
Technology   Manufacturer  Product Model             Capacity [*]
                           type
Linear       Tandberg Data SLR     SLR24             12GB
Recording
             Tandberg Data SLR     SLR50             25GB
Helical Scan Hewlett       DDS3    Surestore DAT 24i 12GB
             Packard
             Sony          DDS4    SDT-S10000/ME     20GB
Technology   Media        Tape Model
             Manufacturer
Linear       Imation      SLR24 12/24 GB
Recording
             Imation      SLR50 25/50 GB
Helical Scan Fuji Film    125M 4mm 12GB/24GB
             Fuji Film    150M 4mm 20GB/40GB
(*.)Native capacity, non-compressed