USB drives: speed & write cycles

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Not all USB drives are created equal
Performance and reliability vary, but getting product specs for flash drives can be tricky.
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 last (theoretically, at least) 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. Without checking reviews or running benchmarks, the only rule of thumb is that the more expensive drives (and those which post performance numbers on the packaging) will tend to be the fastest and, perhaps, last the longest.

The casual user may not ever notice. "For the average user, moving a few files around, or even 20-50MB of data, a slower drive is probably sufficient," says Cameron Crandall, a technology manager at memory vendor Kingston Technology Corp. And while the memory in less expensive drives has a shorter life span than that in more expensive drives, even that is enough to last the lifetime of the drive for an average user.

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

Key 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, and MLC stores two bits of data in each memory cell. SLC is twice as fast as MLC, says Wilkison, with maximum read speeds of about 14 MB/sec. and write speeds of about 10-12MB/sec. Not surprisingly, almost all current USB flash drives are built using MLC memory, since SLC costs about twice as much as MLC.

Users would see the greatest performance difference between SLC and MLC if they were performing many operations involving small files, rather than relatively few read/write operations on larger files, says John Whaley, principal engineer at MokaFive Inc., whose 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. However, this difference won't be of much concern to many users.

Have Your Say

Is the speed of your USB drive important -- or a nonissue?

"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 an SLC-based drive might be worthwhile for a user storing, say, a virtual machine on a flash drive to restore their system after a disaster, according to Wilkison. If the drive began to fail, dropped bits might not be noticed in a photo or music track but could crash the system if they disappeared from a key part of an operating system.

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 the channel is used to its maximum potential.

As with many other design details, it's difficult for consumers to determine which I/O controller is used in a specific drive and which features it provides. While a consumer could try to find out which I/O controller is used in a specific drive and investigate how that controller works, most buyers will, according to Wilkison, have to "extrapolate from the speed" of the drive what type of controller is in it.

No standard

Right now, possibly the only way to know whether you've bought a drive that uses SLC memory is how much it costs -- 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.

However, 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, 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 marketed as a high-performance device and probably priced at the high end of the average for its capacity.

The future

In the first half of 2009, Wilkison predicts, controller manufacturers will begin shipping drives with dual- and even four-channel controllers, which will increase speeds even for slower MLC memory by increasing the number of lanes through which data can be written to and from the memory cells. USB drives combining four-channel controllers with MLC memory will reach speeds of about 60MB/sec. for reads and 30MB/sec. for writes, coming closer to -- but not quite -- saturating the USB 2.0 interface.

For the average consumer for whom price is more important than speed or reliability, any reasonably priced USB drive should do. But if speed or longer life is critical, 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.

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