Super DLT Looks Forward... and Back

by Lowell Rapaport

• SDLT Libraries and Loaders (PDF, 15 K)

The long-awaited next generation of DLT tape drives, the Super DLT, has finally arrived. It's a crucial introduction, as it promises to bring the tens of thousands of DLT tape users to the next plateau of storage density without orphaning their legacy investments.

The first Super DLT drive is the SDLT 220, from Quantum, Milpitas, CA, and it began shipping in May. The SDLT natively stores 110 gigabytes (GB) per tape, and it streams up to 11 Mbytes/sec. The new drives have the same footprint as the DLT tape drives they will eventually replace, and many libraries can be updated with a simple drive swap plus a firmware upgrade. This combined with the SDLT drive's ability to read the original DLT tapes preserves users' investments in tape libraries and archives.

For those not familiar with DLT tape, the format employs half-inch tape wound around a single reel in a 4.125-inch-by-4.1-inch-by-1-inch cartridge. When a cartridge is inserted into a drive, a hook grabs the end of the tape and winds it around a take-up reel permanently installed inside the tape drive. Super DLT shares some of the original format's disadvantages, namely relatively bulky drives and cartridges as well as high power consumption of 43 watts (compared to 15 watts for AIT-2 drives). These disadvantages are mitigated by Super DLT's performance gains.

SDLT's high streaming speed makes it an excellent drive for backup applications and for reading and writing large files. When planning backups, it's important to match the speed of a storage subsystem to that of the SDLT drive. Hard disk drives and RAID systems designed for transaction processing tend to stream data slower than tape drives like the SDLT 220. This results in "shoe-shining" or "shuttling," where the tape drive runs out of data and has to stop and reposition the tape while additional data is picked up from the hard disks. The SDLT 220 addresses this problem by including a large, 32 MB buffer. This is at least twice as large as buffers used in previous generations of DLT tape drives.

Not surprisingly, Quantum has taken advantage of its long experience with DLT to refine the design of SDLT tapes (which will be offered by Quantum as well as many third-party manufacturers). For example, Quantum has abandoned the plastic loop used in the original DLT design to draw the tape out of the cartridge. Instead, SDLT cartridges use a more robust plastic and metal latch. Users who have experienced broken DLT take-up loops will appreciate the change.

SDLT is a completely different tape format from the original DLT in the way it records data. In fact, a non-backward-compatible version of the SDLT has been available since the beginning of the year. The new SDLT 220 adds backward compatibility with earlier DLT tapes by adding a conventional DLT drive head in the tape path. Backward compatibility is read-only and extends back only to DLT-4000/7000/8000 and Benchmark DLT1 tape formats.

Super DLT's biggest rival is the Ultrium tape format introduced last year by IBM, Hewlett Packard and Seagate. Each 4.1-inch-by-4-inch-by 0.8-inch cartridge natively stores 100 GB, and the drives can record at 15 Mbytes/sec. Ultrium is an all-new format that is not backward compatible with any previous generation of tapes, though it is designed to fit in the same, half-inch libraries as DLT and Super DLT tape drives.

Is SDLT right for you? If you already have an installed base of DLT libraries and cartridges, the decision is easy. "DLT's installed base combined with Quantum's established reputation should permit Super DLT to gain acceptance in the marketplace much faster than other tape formats without established track records," says Jim Jenkins, director of product marketing at Overland Data, San Diego, a library manufacturer that is now installing Quantum's SDLT drives. "It's a less risky move."

Most of the other companies in the half-inch tape market agree that SDLT has an edge over Ultrium due to its backward compatibility. However, it's not completely cut and dried. "It's hard to tell which format will eventually win out," says Steve Whitner, marketing manager for ADIC, a library and storage software manufacturer in Redmond, WA. "They're both good technologies with backing from big companies, and they both provide compelling capacity and performance."

Ultrium drives have a 36 percent edge over SDLT in terms of maximum speed, while SDLT cartridges have 10 percent more capacity. Ultrium tapes are thinner than their SDLT counterparts, permitting more cartridges to be placed in a library; therefore, the total storage capacity of Super DLT and Ultrium libraries generally balances out.

Ultimately, the decision over which tape technology to choose will probably not come down to specifications like capacity and speed. Competing tape technologies regularly leapfrog each other on these specifications. Wait a few months and there will always be a faster and higher-capacity tape.

More likely, your decision will come down to backward compatibility, your experience with the vendors and long-term costs like the price and availability of media, power consumption and space requirements. On several of these grounds, Quantum's Super DLT is a competitive product worthy of consideration.