Not all USB drives are created equal

By Robert L. Scheier, Computerworld |  Personal Tech, flash media, USB

Most USB 2.0 flash drives look the same, but that doesn't mean they perform the same.

Differences in the type of memory and, to a lesser extent, the type of I/O controllers used by USB drives can make one device perform two or three times faster and (theoretically, at least) last 10 times longer than another, even if both sport the USB 2.0 logo.

Unfortunately for the average user, there are no accepted industry standards or certifications to judge what's inside a USB 2.0 flash drive on a store shelf. Aside from checking reviews or running benchmarks, the only rule of thumb is that the more expensive drives (and those with their performance numbers on their packaging) tend to be the fastest and, perhaps, the ones that last the longest.

The casual user may never notice a difference. "For the average user moving around a few files, or even 20 to 50MB of data, a slower drive is probably sufficient," says Cameron Crandall, a technology manager at memory vendor Kingston Technology Corp.

And although the memory in less-expensive drives has a shorter life span than that in more-expensive drives, it's long enough to last the lifetime of the drive for an average user.

But these differences can matter if you're storing large amounts of data, using the drives to store critical information or using a USB drive to supplement system memory via the ReadyBoost feature in Windows Vista .

Performance Factors
The USB 2.0 standard supports a maximum throughput of 60MB/sec., although "nobody's pushing that limit" with the flash memory used in current USB drives, says Pat Wilkison, vice president of marketing and business development at Stec Inc., a Santa Ana, Calif., manufacturer of memory and storage products.

The single biggest factor in USB drive performance is whether it contains one of two types of memory: SLC (single-level cell) or MLC (multilevel cell). SLC stores one bit in each memory cell, and MLC stores two bits in each cell.

SLC is twice as fast as MLC, with maximum read speeds of about 14MB/sec. and write speeds of about 10- to 12MB/sec., says Wilkison. Almost all current USB flash drives are built using MLC memory, however, since it costs about half as much as SLC.

Users would see the greatest performance difference between SLC and MLC if they were carrying out many operations involving small files rather than relatively few read/write operations on larger files, says John Whaley, principal engineer at MokaFive Inc. His company's virtualization software makes it possible for virtual machines to be stored on USB flash drives.

SLC memory also lasts about 10 times as long as MLC, says Crandall, which means one cell in an SLC-based USB drive should last for about 100,000 cycles of writing and erasing data before it fails. This difference won't concern most users.

Have Your Say
How important is the speed of your USB drive?

"If you save a file out to your USB drive and use 100 bytes, you're probably not going to write to those same 100 bytes again for a long time, unless for some reason you decide to delete that file or change it," says Crandall.

As a result, the usable lifetime of the device will extend long beyond the customary five-year warranty, says Mike Sager, vice president of public relations at Kingston.

When USB drives do begin to fail, they do so one cell at a time, not across the board, says Crandall. This is why Wilkison says that an SLC-based drive might be worthwhile for a user who stores a virtual machine on a flash drive to restore a system after a disaster, for example. If the drive began to fail, dropped bits might not be noticed in a photo or music track, but if they disappeared from a key part of an operating system, that could cause a crash.

There are features in the I/O controller that can boost performance in USB drives as well. One is the use of multiple channels to simultaneously move data to and from memory, says Brad Anderson , director of product marketing at USB flash drive vendor Lexar Media Inc. Another, he says, is interleaving, which intermixes data flows to and from multiple flash memory chips within the drive to ensure that the channel is used to its maximum potential.

It's difficult for consumers to determine which I/O controller is used in a specific drive and which features it provides, but this is true of many design details. Most users will have to extrapolate from the speed of the drive what type of controller is in it, Wilkison says.

No Standard
Right now, the best indicator of SLC or MLC memory is price. The more expensive the drive, the more likely it is to have been built using SLC.

There is one way to judge the quality of a drive besides the price: the Windows ReadyBoost logo, which indicates that the flash drive can be used to supplement system RAM and thus speed the performance of Windows Vista-equipped PCs.

The minimum specifications for Windows ReadyBoost are just 2.5MB/sec. for random reads of 4KB of data, and 1.75MB/sec. for random writes of 512KB of data, however, which Crandall says are typical of lower-priced and lower-speed USB flash drives.

Customers who want the maximum benefit from Windows ReadyBoost should opt for a USB drive that is marketed as a high-performance device and probably priced at the high end of the middle range for its capacity.

For the average user, for whom price is more important than speed or reliability, any reasonably priced USB drive should do. If speed or longer life is critical, however, look for drives advertised as high-performance, do your research online and expect to spend more.

But even then, you can't be absolutely sure you're getting more speed for your money.

Scheier is a freelance technology writer based in Boylston, Mass. He can be reached at bob@scheierassociates.com.

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