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DDS-3: DAT Gets Bigger and Better

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DDS-3, a Leap in Capacity and Performance

When it comes to backing up network servers, DAT has long held the championship title. DAT is the most popular solution for protecting data on PC LAN servers, workstation servers and many enterprise servers. Information Technology professionals depend on DAT for its superior combination of reliability, compatibility, performance and affordability.

DAT has certainly marked its territory with a vast installed base. In 1996, more than 1.45 million DAT drives were shipped, according to industry analysts IDC.

To keep DAT in the number-one position in the high-end tape battle, the Digital Data Storage (DDS) Manufacturers Group, made up of numerous leading storage manufacturers including Seagate, began to establish the outline specifications for DDS-3 in early 1992, the same time that specifications for DDS-2 were finalized. Final specs for DDS-3 were established in 1995. As the latest DAT standard, DDS-3 takes the technology to an entirely new level of performance and capacity with the ability to store up to 24 Gbytes of data in compressed mode (12 Gbytes native) compared to the 8 Gbytes maximum in compressed mode previously offered by DDS-2. DDS-3 data-transfer rates are up to twice as fast as DDS-2 with speeds up to 2.2 Mbytes per second.

Compatibility and Interchangeability

No one likes to change technologies or standards when a lot of time and money have been invested. Ask anyone who had a huge collection of their music on vinyl how they felt the first time they walked into their favorite record shop only to find nothing but tapes and CDs!

IT professionals with volumes of DDS-2, DDS and DDS-DC DAT cartridges do not want to spend the time and money to convert all that data into a different technology format. Now that DDS-3 is on the scene, the issue is resolved. DDS-3 mechanisms like Seagate’s Scorpion 24 drives will read all previous formats of DAT cartridges so that any data can be retrieved easily and used in the most up-to-date backup device on the site.

Interchangeability is also a key purchase consideration. It’s comforting to know you’re storing your data in a format that is so widely used. When corporations merge, for example, it’s a good bet that both will be using DAT to back up their data, ensuring an easy transition.

DDS-3 Media—Powerful Stuff

Seagate and other members of the DDS Manufacturers Group wanted to maximize the potential of DAT media by using a higher linear track density and a more space-efficient track format. After investigating all relevant technology areas including channels, tracking schemes, error-correction codes and track structure, the group was able to devise a tape with a linear bit density twice as great as DDS-2 and a track format efficiency about 1.5 times as great. These improvements are what make it possible for DDS-3 to hold up to 24 Gbytes of data, three times as much as DDS-2.

DDS-3 tape uses advanced Metal Particle media (MP++) which is even more durable than DDS-2 because it incorporates a ceramic layer that coats each magnetic particle to provide protection from oxidation. Overwrite and distortion performance is also greatly improved due to a dual-coating in the media that provides an extremely thin magnetic layer. This technology is widely used on hard discs and with the advent of DDS-3 it is now available on tape.

What Makes DDS-3 Possible?

One of the keys to the implementation of DDS-3 was the DDS Manufacturers Group’s ability to find a way to double linear-bit density without cutting in half the size of the gap in the magnetic write and read heads. At high-bit densities, the read-back signals from adjacent magnetic flux transitions on the tape interfere with each other to a degree, depending on the size of the head gap relative to the distance between transitions. Instead of shrinking the head gap to minimize this interference, the read-back channel characteristics are changed to compensate for it.

The signal from the head is determined by pairs of bit cells, instead of single-bit cells. However, by knowing the rules that govern the allowable sequences of bit cell states and what signals to expect from given bit-cell pairs in those sequences, an advanced read-back channel can reconstruct the individual bit-stream orientations and recover the original recorded bit stream. Such channel schemes, which are known as partial-response maximum likelihood (PRML), are now realizable in LSI circuitry at costs that are acceptable for DDS-3 products.

The second key to developing the DDS-3 standard was the group’s ability to obtain an adequate signal-to-noise ratio (SNR) at 122 kbpi from media that uses the same metal powder (MP) coating technology used in DDS and DDS-2. The magnetic performance is further enhanced in the new DDS-3 media, known as MP++. Seagate also designed a flying signal preamplifier (see Figure 1) that is attached to the head cylinder, offering additional increases in signal quality and improvements in effective SNR. Since the preamp is attached directly to the drum, a cleaner reading is achieved than if the signal were read from the printed circuit board.

Figure 1. Seagate DDS-3 Flying Preamplifier


Finally, a track-following scheme called timed tracking was developed. This generates input to the servo system by measuring the time it takes the heads to scan from a reference point to a known point along the track. A deviation from nominal time indicates a deviation from following the centerline of the track. Unlike the automatic track format (ATF) scheme of DDS and DDS-2 (See Figure 2), timed tracking does not rely on the read head picking up tone cross talk from dedicated areas in the recorded track.

Figure 2. ATF Tracking


The use of timed tracking (See Figure 3) allows the ATF areas recorded on tape to be removed, which makes more of the track available for user data. Also, the subcode information has been compressed into less space and redistributed along the track. A more space-efficient Reed-Solomon code has been chosen for the C1 error-correction scheme. Finally, the user data has been packed into fewer, larger blocks; this reduces the total block header overhead. The result of all these changes is a 50 percent increase in the proportion of the track that stores user data.

Figure 3. Timed Tracking


Drive Features

  • Bus architecture. The DDS-3 drive design incorporates a triple bus architecture that allows the major drive elements—control, interface, formatting, memory and microprocessors—to operate with increased levels of independence from one another. This increases the efficiency of the system functions and significantly improves the reliability of the command sequences.
  • Error corrections. The drive employs sophisticated error-recovery techniques, including the C1 and C2 Reed Solomon error-correction codes specified in the DDS format standards. C1 ECC is (32,28) Reed Solomon code and can detect and correct errors in any two symbols, or it can correct four symbols where the error location is known. C2 ECC is (32,26) Reed Solomon code and can correct errors up to three symbols long, or six symbols when the error location is known. These codes are stored in the same track as the data.

In addition, a third level of error-correction code covered by the DDS format is used. C3 allows any two tracks in a group to be corrected, and is used only when a raw data error is too big to be corrected by C1 and C2. C3 code is stored in an extra frame at the end of the twenty-two frames of data in each group.

  • Data Compression. Data compression in the DDS-3 drive is based on reducing the redundancy that occurs naturally in data streams of text, graphics, code and other data. Reducing or eliminating such redundancy before recording the information to tape significantly increases the amount of data that can be recorded on a given amount of tape.

Data compression causes repeated strings of data to be recognized and replaced by symbols or code words that encode the strings or point back to the original occurrence of the string. In this way, data compression uses fewer characters to represent the original data.

Seagate’s DDS-3 drive uses the DCLZ (Data Compression Lempil Ziv) standard to compress and decompress data, which ensures compatibility with other manufacturers’ DAT drives.

  • Read-While-Write. The DDS-3 drive’s read-while-write or read-after-write (RAW) technique provides a means of verifying that host data was written on the tape correctly by reading it on-the-fly. After each frame is written by a write head, it is examined by a read head to determine whether or not it is correctly recorded.

If a frame is identified as "bad", it is rewritten further along the tape. The bad frame is not necessarily rewritten immediately. It can be rewritten after three, four or five frames have been written. Any frame can be rewritten multiple times to provide for skipping over bad areas on the tape. The maximum number of times a rewrite sequence can occur is 128.

The Seagate Difference—Scorpion

Seagate designed its Scorpion 24 and Scorpion 8 (newest DDS-2 drive) with a number of innovations including higher-performance heads, improved read-channel performance and 50 percent fewer electronic components than previous Seagate DAT drives for increased drive reliability. The Scorpion 24 and Scorpion 8 are also energy savers with a 42 percent reduction in power consumption from previous drives, resulting in lower thermal dissipation.

In addition, Seagate sought to reduce end-user maintenance and improve reliability when we designed the Scorpion 24. That’s why we implemented a unique, new self-cleaning device. A preventive-cleaning operation kicks in when information monitored by the tape head interface triggers the cycle, and a reactive-cleaning cycle automatically kicks in if a data retry occurs. The mechanism is made of poron foam with a mypox cleaning component that has proven to efficiently remove dust and other particles that can cause errors.

The Seagate Scorpion is available in 3.5-inch internal, 5.25-inch internal and external form-factors.

See Also