The promise of superfast access speeds has been fulfilled by solid state drives using NAND flash memory, but it comes at a high monetary cost. Many enthusiasts, prosumers, and corporate users have already adopted SSDs as hard drives are typically the primary bottlenecks in a computer system. While CPUs and RAM measure their access speeds in nanoseconds, traditional magnetic-based hard disk drives are still measured in milliseconds.

The primary advantage for HDDs is the low cost of production, even for faster 7200 and 10k RPM drives. Samsung and Seagate tried before to bridge the gap between low-cost HDDs and fast SSDs using a hybrid hard drive, combining a single magnetic platter with a small amount of NAND flash memory. The NAND would act as a cache, similar in theory to a small scale tiered storage solution like that used by many corporations for their datacenters.

However, the experiment failed. NAND was still too expensive in 2007, and the small amount that was used proved insufficient. Performance turned out to be worse in some situations, and the capacity of Seagate's sole model was soon overtaken by other products with higher areal density.

Now, three years later, Seagate has learned its lesson with its all new Momentus XT. The company calls it a Solid State Hybrid Drive, and it will be available exclusively in a 2.5-inch form factor. There are 250, 320, and 500GB models, all of which feature 32MB of DRAM cache and a 4GB single-layer cell NAND flash cache. There will unfortunately not be any 6Gbps SATA support, despite the XT moniker.

The secret sauce this time is what Seagate calls "Adaptive Memory". The firm has developed new algorithms based on their years of research and producing firmware for regular drives. These algorithms monitor data access transactions over time, and will place a copy of the most frequently accessed data (such as Windows system files) onto flash storage. A table also keeps track and counts of how frequently data is used in order to prioritize it for retention and caching.

This is similar in concept to Microsoft's ReadyBoost, but uses much faster SLC rather than the sluggish commodity NAND that ended up being used in USB flash drives and SD cards. The algorithms are also much more advanced, as is the garbage collection and firmware. Seagate developed its own proprietary NAND flash controller specifically for the Momentus XT.

This also means that the Momentus XT is also operating system agnostic, and can be used with Unix/Linux environments and MacBooks.

Seagate insisted on using the flash as a cache instead of primary storage for additional reliability. Their tests show that over 250GB of data a day could be written to the NAND for 5 years and it would still function.

Although the Momentus XT isn't as fast as an SSD, Seagate thinks that it will be close enough that its customers won't be able to notice the difference qualitatively. While most consumers will notice the difference between a 5400RPM drive and a 7200RPM drive, they might not necessarily notice the difference between a 7200RPM and 10k RPM drive, an argument that the company has used before as a justification for not producing a 10k RPM consumer drive.

To continue the example, Seagate likens the Momentus XT to a 7200RPM drive and SSDs as 10k RPM drives; while the SSDs are much faster, qualitatively consumers won't notice the difference. The company expects that it will be able to hold off the SSD onslaught for a couple of years with this strategy, despite the lowering cost of NAND flash memory. In fact, as NAND flash prices drop due to the introduction of new process nodes, Seagate will be able to fit more NAND into the same space and offer even greater performance.

The first OEM to adopt the Momentus XT will be ASUS for their ROG G73JH gaming laptop, which will feature two of the drives. Seagate will also be shipping Momentus XT drives out to the channel this week for retail distribution.

Two reviews of the new Momentus XT can be found here and here.