Weighing the risks of 1TB drives

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One terabyte SATA and SAS drives are here. And when larger capacity drives come to market, people always cram them with data. That being said, what are the implications of storing that much data in one place? The risk is the same as it ever was: data loss. However, storing a terabyte of data in one place certainly makes the risk of data loss more acute. Experts agree that it's very important to match these drives to the right tasks. For now, it looks like that's still secondary storage.

"When it comes to nearline storage, its all about capacity," says Willis Whittington, senior manager of market development at Seagate Technology. And while Whittington was unwilling to comment on specific release dates for Seagate disks larger than 1TB, he did say that "You can expect to see a 40% to 50% increase in aerial density from generation to generation."

With disks this large, rebuild time for failed drives is a legitimate concern. That's why RAID 6 is often recommended, as it protects against dual-disk failure. However, "RAID 6 is duct tape," says Greg Schulz, founder and senior analyst at StorageIO Group, Stillwater, MN. "It's RAID 5 with an extra parity disk. Basically you're saying 'It's going to take even longer to rebuild, but at least I'm protected.'"

RAID 6 is an excellent added layer of protection in operations in which you would typically use RAID 5, says Schulz. However, you don't want to rely on RAID 6 for really demanding primary storage tasks, says Schulz. "If you have to depend on RAID 6 because you can't tolerate exposure [associated with extended rebuild times], you probably want to take a look at your choice of drives and the quality of the drives."

SATA disks have a reputation for being a little more wobbly at the spindle, and vibration has been a major reliability factor. But, says Whittington, this is becoming less of a concern, especially at the high end. "It's all about the fine tuning," he says. "The drives themselves can't do anything about reducing vibration because that's an external thing. What they can do is react to the vibration. And that's in the servo system."

In high-end drives, vibration tolerance is accomplished by programming the read/ write heads to compensate for vibration. "However, to do this you have to spend some money," says Seagate's Whittington. "We have to put something in the drives that's going to increase [the] cost for users."

In addition, some RAID controllers do preemptive rebuilds as a way of preventing disk failure. Controllers monitor the drives and then take a drive out of operation if there are signs of abnormal operation. This feature is far from ubiquitous, and not all RAID controllers perform this function the same way. According to StorageIO Group's Schulz, some vendors' firmware is overly sensitive, which can trip off false rebuilds; this can have a negative impact on performance due to the processing demands of the rebuild. "You really have to talk to other users and research the reputation of different systems," says Schulz.

Power consumption is a concern with drives of any size. The so-called "green storage" movement, which many vendors are scrambling to associate themselves with, promises products that expend less energy, generate less heat and generally make everyone feel good. For example, Seagate Technology offers a 1TB drive with PowerTrim, a feature it claims reduces power consumption at the drive level.

"If you're not using something, why power it?" says Whittington. "If you're doing a write, can you shut down all the read electronics and circuitry associated with that?" Depending on the app, in theory, this could translate into a significant reduction in power consumption.

Hitachi Data Systems and Western Digital also offer 1TB drives that accomplish power reduction using a variety of techniques, including idling the disks. These features could potentially increase reliability by reducing the amount of heat the drives are subjected to.

In a related note, drives that use solid-state memory as large onboard cache could emerge down the road. This additional memory would allow the drives to perform many operations while the disks sit idle, further reducing power consumption. [Note: Check out "Solid-state storage finds its niche"]

"Anything that involves flash, that makes sense; we'll be driving it for sure," says Whittington. However, according to Jerome M. Wendt, lead analyst and president at DCIG, this technology is most likely a long way off for most users.

--Andrew Burton

This was first published in November 2007

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