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Seagate's Momentus XT hard drive

A hybrid for the masses?

When Windows Vista arrived more than three years ago, Microsoft's then-flagship desktop operating system brought with it a number of new technologies designed to improve performance. SuperFetch promised to deliver performance gains by caching frequently access data in unused portions of main memory. ReadyBoost took a similar approach, but rather than filling free memory opportunistically, it tapped the unused capacity of USB flash drives. Then there was ReadyDrive, which offered support for hybrid hard drives that combined flash memory with traditional mechanical platters.

In theory, hybrid hard drives make perfect sense. Flash memory offers essentially instantaneous seek times that are at least an order of magnitude quicker than the fastest mechanical hard drives, but it's also quite expensive. That's why SSDs have relatively low capacities and rather unattractive cost-per-gigabyte ratios. Traditional hard drive platters can offer substantially more storage capacity at a much lower price, but their inherent mechanical latency makes for much lengthier access times. Meld those technologies together, and you should be able to enjoy the best of both worlds.

I had high hopes for hybrid hard drives after Vista's launch. However, that was to be the last I heard about ReadyDrive. Samsung and Seagate did end up bringing hybrid drives to market, but neither company's offerings gained much traction. In retrospect, I'm not surprised. Being hitched to the most reviled operating system since Windows ME surely didn't help ReadyDrive's chances. With most users and businesses happy to stick with Windows XP, the potential market for hybrid drives may have ended up being much smaller than initially hoped.

The hybrid storage concept didn't die out as ReadyDrive faded into obscurity. As the price of solid-state drives declined, PC enthusiasts began rolling their own hybrid storage setups by combining low-capacity SSDs with mechanical hard drives. Such configs really do deliver the best of both worlds. The SSD can be used to house OS and application files that need to be accessed quickly, while the mechanical drive provides ample capacity for data that just needs to be stored.

The do-it-yourself hybrid approach might work well for desktops, but few notebooks can accommodate the two drives required. Fortunately—and wisely considering the market's shift toward notebooks—Seagate didn't abandon the hybrid concept entirely when it gave up on ReadyDrive. Instead, the company set out to devise an OS-independent hybrid technology that appears in the new Momentus XT, which combines 500GB of 7,200-RPM mechanical storage with 4GB of flash memory. Naturally, we had to sample this hybrid redux for ourselves.

Beyond ReadyDrive
Before exploring the new hybrid direction Seagate is taking with the Momentus XT, it's worth delving into the concept's ReadyDrive roots. As one might expect, ReadyDrive was designed to populate a hybrid drive's flash memory with frequently accessed data. Data associated with system startup and hibernation are also loaded into the flash because, according to Microsoft, the system often waits on traditional hard drives during those tasks.

There's more to ReadyDrive than speculative caching to improve performance, though. Microsoft wanted hybrid drives to offer power savings, so ReadyDrive can spin down a drive's platters and operate exclusively out of its flash memory. If a read request can be serviced by the flash's contents, there's no need to wake up the platters. Writes can also be cached in the flash in this low-power state, with the platters springing into action only if the flash doesn't have enough free capacity to accommodate a write request.

When Microsoft architect Ruston Panabaker presented ReadyDrive at WinHEC in 2006, he called for a minimum flash capacity of just 50MB. 120-256MB was recommended as the minimum necessary to realize "significant" performance and power consumption benefits. That might not sound like much, but keep in mind that Vista was in development at a time when flash memory was considerably more expensive than it is today. During that era, we reviewed an IDE solid-state drive that cost a whopping $320 for just 8GB. Fast-forward to the present day, and $320 will get you a Kingston SSDNow V+ SSD with 16 times the capacity.

According to Seagate, ReadyDrive tried too much to be all things to all people. The hard drive maker wasn't crazy about being tied to a single operating system, either. Even if Vista hadn't been a pariah, hybrid hard drives are particularly appealing in the mobile world, which just happens to be filled with a whole lot of MacBooks running OS X. Besides, Seagate contends that a hard drive can make better decisions about what to put in its flash because the drive knows more than the operating system about how data is arranged on the disk.

The Momentus XT looks unassuming, even up close