CHOOSING A TAPE TECHNOLOGY.

Can reliability come at a reasonable price?

The steady increase in hard drive capacities has resulted in more and more data being stored to disk, which means more and more data must be backed up. While tape is still the most economical and popular way to archive data, the recent explosion of new tape technologies has made choosing the right backup device a more involved process.

Before selecting a tape drive IT managers should carefully evaluate not just individual product features, but also the technology each product is based on. The choice will impact the organization for years to come.

Many IT managers have chosen to employ legacy tape technologies like DAT and Travan due to their long histories and affordable prices. However, buyers should be cautious of older technologies that may have reached the end of their lifecycle. The whole purpose behind archiving is to be able to go back months and even years later to retrieve data. Nothing could be worse than having a warehouse full of tape cartridges based on a technology that no one supports anymore. The DAT DDS drives are a good example of a technology that has likely reached its last generation. Within a few years, DDS will likely no longer be produced.

The developers of new tape technologies always tout how much faster and higher in capacity their drives are than the competition. However, a major problem is being overlooked--the price for these new tape devices is often more than the servers they are backing up. Choosing a $5,000 DLT drive to back up a $1,000 entry-level server just doesn't make sense.

ADR Technology Offers Reliability, Capacity At An Affordable Price

ADR (Advanced Digital Recording) technology helps bring entry to mid-range level server-to-backup price ratios back in line, by offering the reliability and capacity of a new tape technology at a subthousand dollar price point. ADR is based on an eight-channel array head technology, which provides the ability to read or write eight tracks of data simultaneously. The data reliability is one error in 1019 bits, which is 10,000 times better than the common tape standard of one error in 1015 bits.

The improved reliability, capacity, and performance of ADR technology is possible at a low cost due to four separate tape storage innovations: buried servo signaling, multi-channel recording, true variable transfer rates, and enhanced data integrity.

Buried Servo Signaling Gives Precise Tape Position Information

ADR technology's biggest innovation is an advanced buried servo system in the media that contributes to the highest data reliability levels of any entry to mid-range server backup tape device.

The buried servo system ensures data integrity by guaranteeing accurate tape tracking. During the manufacturing process, the low frequency proprietary servo signal is Written deep within the media layer. During customer use, high frequency user data is written on top of this servo signal. The servo signal is a series of sine waves which provides precise feedback to control the position of the read/write head on each of the 192 data tracks across the width of the tape. During the read or write process, the head detects the sine waves' amplitude and phase to determine the head's lateral location on the tape and makes corrections to keep it on track.

While the head is writing, the embedded servo signal is also used to ensure data integrity. If the head detects a perturbation on the servo signal due to an off-track condition, or degradation of the signal due to loss of head to tape contact or damage to the media surface, the drive will immediately stop writing to ensure that data on adjacent tracks is not overwritten. The drive then initiates a rewrite operation to ensure that the data is written correcfly.

Multi-Channel Recording Increases Media Life

ADR technology allows the drive to reduce the speed of the tape under the head by a factor of eight when compared to single track drives. The lower tape speed results in gentle tape handling that improves the life and reliability of the drive and media. At slower speeds, less power is required from the host and less friction is generated from media to head contact, resulting in a lower overall temperature and less media wear. Lower speeds also allow ADR drives to run at the mid-40 decibel sound range--barely detectable in an office environment.

By reading eight tracks simultaneously, all 192 tracks of an ADR cartridge can be read in 24 end-to-end passes. A typical single-track drive requires 106 end-to-end passes to read all 106 tracks on its media. This gives ADR media a significantly longer life expectancy since tape life is generally expressed in terms of total end-to-end passes.

True Variable Transfer Rates Optimize Throughout

A traditional issue with tape technology has been that systems supply data at variable rates while tape drives write at a fixed rate. To compensate, a tape drive does one of two things when data is not reaching the drive fast enough: 1. In a process commonly known as back hitching, the drive will stop, back up, reposition itself, and continue writing when more data arrives. This continuous start-stop cycle can cause unnecessary drive and media wear. This degradation of tape tension can cause catastrophic data loss. 2. Other tape drives reduce their transfer rate when the data transfer rate slows. This causes degradation of performance because typically the drives cannot increase speed again until they reach the end of the tape and reverse. ADR technology, however, allows infinitely variable tape speeds that can support data transfer rates anywhere between .5MB/sec to 2MB/sec.

Improved Error Correction Enhances Data Integrity

Older tape technologies have to run a separate verification pass after the data is encoded--effectively knocking transfer speeds down to half the nominal speed and cutting the tape lifespan in half--to ensure data reliability. Many of the current high performance tape drives use a Read-While-Write feature that enables them to read, to verify data integrity, while writing. This requires the incorporation of dual gap heads with extra read verify circuitry resulting in increased product costs of approximately 25% and of course extra complexity.

The newly-introduced ADR uses its servo signal in an innovative way to give the same level of reliability as Read-While-Write without the traditional hardware complexity. With ADR media, the servo signals lie underneath the data and are used as an indicator for media integrity. As the head writes data, it also is detecting the servo signals. If the servo signals are absent, the system maps the region as defective and re-writes thq data. This approach allows ADR technology to achieve data reliability without sacrificing speed, tape life, or price.

Tape devices improve data integrity by scattering ECC (error correction code) bits across the data track. Mathematical algorithms can then use the ECC to reconstruct missing data bits. ADR technology's superior error rates are a result of its ability to read and write all eight tracks at once, which makes it possible to distribute error correction code spatially, that is, both horizontally and vertically across eight tracks. This allows data to be recovered from a tape with wide scratches across its width or even with a single track wiped off the entire length or the tape.

Duncan Furness is the director of R&D at OnStream inc. (Longmont, CO).