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Tape bulks up to keep pace with disk

Is the amount of data a tape can store more important than how fast it can write it? Judging by some of the new drives hitting the market, tape vendors seem to think so. From one generation to the next, tape platforms are routinely doubling in capacity, while their performance is increasing at a much lower rate.

For example, Quantum's new DLT-V4, the successor to DLT VS160, has twice its predecessor's capacity at 160GB native, but has only increased its native throughput by 25%--from 8MB/sec to 10MB/sec. Similarly, Quantum's roadmap for the next generation of Super DLT drives, the DLT-S4, will see capacity swell by 166% to 800GB, while performance increases by only 67% to 60MB/sec.

The ability to hold more data is particularly important for tape drives that ship with entry-level servers, says Mark O'Malley, Quantum's product marketing manager. That's because today, most entry-level servers can ship with several hundred gigabytes of internal disk drives. Furthermore, Quantum's market research shows that low-end server buyers have very little in the way of tape automation, prefer their backups to fit on a single cartridge and don't like to do incrementals.

That's bad news for low-capacity tape cartridges like the ones that dominate the entry-level market. Sony's DAT 72, for example, has capacity for 36GB of data; Travan 40 has only 20GB. At these rates, "they are losing touch with the capacity points of your average server," says O'Malley.

Many storage administrators have already given up using tape to back up entry-level servers, and have started backing up to large external USB disk drives, says O'Malley. To combat that trend, a vendor's only chance at salvaging low-end tape drives is to make them not only bigger, but cheaper. Quantum's DLT-V4 has a list price of $999, with a target media price of $39 per cartridge. For servers that don't have a SCSI host bus adapter in them, you can choose the drive with a native SATA interface to further cut costs.

In the midrange, capacity is also king. Tape drive and automation vendor Exabyte sells equal numbers of autoloaders based on LTO-2 and LTO-3, says Kelly Beavers, VP of product marketing. "It's safe [to assume] that the biggest justification for the LTO-3 version is the extra capacity," he says, and not the extra throughput. LTO-2 is rated to 200GB and 40MB/sec native vs. 400GB and 80MB/sec for LTO-3.

Unfortunately, most shops can't take advantage of the extra speed they're buying from the latest and greatest tape drives. "There are very few shops--I'd venture to say there aren't any--that can reliably stream 80MB/sec," says Beavers. The practical limit these days is probably around 30MB/sec to 50MB/sec, he says. Not being able to feed a drive with enough data can cause the drive to "shoeshine" or "backhitch"--stop, rewind and start again--which can slow backups and cause unnecessary wear and tear on the drive.

But ultimately, you can't increase capacity without also increasing performance, says Charlie Andrews, director of storage marketing at IBM. "The two go hand in hand," he says. That's because if you're increasing capacity by increasing bit density on the media, "as the capacity goes up, the data rate goes up," notes Andrews.

Or does it? Last month, IBM introduced the high-end System Storage TS1120--the successor to the 3592 drive--which runs at 100MB/sec and can hold 500GB of native capacity. Next year, Andrews says, IBM will announce a new version of the drive in which speed remains the same, but capacity doubles to 1TB. "There is a strong focus on capacity, as tape has the lowest cost of any class of storage," says Andrews.

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