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Seagate's Momentus XT 750GB hybrid hard drive


Adaptive memory enters its second generation
— 12:26 AM on November 29, 2011

For most folks, hybrids are probably the future of PC storage. Flash memory's near-instantaneous access times are a performance benefit too good to pass up. At the same time, NAND flash's relatively high cost per gigabyte, combined with our ever-growing thirst for storage capacity, ensures mechanical platters will remain in a supporting role for the foreseeable future. Music and movie archives aren't likely to benefit from the wicked-fast access times of solid-state storage, anyway.

On the desktop, cobbling together a hybrid solution is as easy as putting a hard drive and an SSD in the same system. Users are free to manage the distribution of data between the two or to rely on third-party software to treat the SSD as a cache for the hard drive. Those approaches work just as well in the mobile world, but only in notebooks that support dual drives, which are relatively rare outside hulking desktop replacements.

For much of the notebook market, the only way to enjoy a hybrid storage configuration is with a single drive that combines flash and mechanical components inside the same 2.5" form factor. Seagate has been making such drives for several generations now, and it claims to have sold more than a million of 'em over the years. Most of those hybrids hailed from the original Momentus XT family, which debuted last summer.

Unlike Seagate's previous hybrid efforts, which relied on the ReadyDrive caching mechanism built into Windows Vista, the Momentus XT took matters into its own hands with an OS-independent caching scheme run entirely within the drive itself. The end result melded 500GB of mechanical storage with 4GB of flash memory dedicated to caching frequently accessed data. Today, that first-generation Momentus XT is being replaced by a new model that combines higher-capacity platters with faster flash memory—and 8GB of it. We've taken the new hybrid for a spin to see if it can hang with not only its mechanical competition, but also the droves of solid-state drives that have flooded the market over the past year.

Momentus XT, take two
Like its predecessor, the new Momentus XT uses its flash memory solely as a read cache. Incoming writes from the host bypass the flash completely. The only time data is written to the flash is when the drive's caching scheme, dubbed Adaptive Memory, is populating it with data from the mechanical platters. Third-party caching solutions like Intel's Smart Response Technology allow flash storage to cache both reads and writes, and Seagate indicates it may move in that direction with its own hybrids. For now, though, the Momentus XT's flash component will only be capable of speeding up the drive's read performance.

Seagate's Adaptive Memory caching tech targets frequently accessed data at the block level rather than the file level. To better take advantage of the new drive's larger allocation of flash memory, the caching scheme has received a number of tweaks that Segate has wrapped up in a marketing name: FAST Factor. The most interesting of these tweaks may be the changes to how the caching system treats Windows files associated with the boot process.

Dubbed FAST Factor Boot, this new approach aims to accelerate the Windows boot process whether you're firing up the OS for the first time or rebooting after months of uninterrupted activity. With the old Momentus XT, Adaptive Memory needed a few boots to learn which OS-related data to cache. Using the drive for long enough without rebooting had the potential to push that data out of the cache, reversing any previous boot-time benefits. With the new XT, a segment of the NAND has been reserved solely for OS data related to the boot process. That section of the flash is populated as Windows is installed to the drive, which should speed things up starting from the first boot. If Windows is being installed via a drive image, the XT will need a couple of boots to determine what to put in the roped-off section of its cache. As with clean installs, that data won't be kicked out of the cache between boots.

On our sample drive, the cache is an 8GB SLC NAND chip made by Micron. Seagate says it will be sourcing flash from multiple suppliers and that the chips could be configured with either 4KB or 8KB flash pages. The original XT uses smaller 2KB block sizes, but like the newer drive, it's also based on SLC-type flash. SLC memory typically has 10 times the write-erase endurance of the MLC NAND chips found in consumer-grade SSDs, making it ideal for caching applications. The 311 Series SSD Intel designed for Smart Response Technology also uses SLC NAND, albeit 20GB of it.

The Momentus XT's 8GB cache is still double the amount of flash available in the original drive. The NAND is faster, too. Seagate claims the new XT can read from its cache at 180MB/s, a 40MB/s improvement over the first model. Both hybrids can purportedly transfer data from the platters to the flash at 100MB/s.

Those speeds might not sound all that impressive in the context of modern SSDs, but keep in mind that the Momentus XT has only a single flash chip. Seagate says it's using a custom bridge design to extract as much performance as possible from that solitary chip. However, the Momentus doesn't have anywhere near the parallelism of modern SSDs, which use multi-channel controllers to address flash arrays typically made up of 8 to 16 NAND chips.

The only other memory on the Momentus XT is a 32MB Samsung DRAM chip that's used as a traditional drive cache. The DRAM chip isn't any bigger than the one used on the original Momentus XT, but Seagate says the new drive's move to a 6Gbps Serial ATA interface will give the cache a little more room to stretch its legs. The XT's flash component isn't quick enough to benefit from the faster SATA pipe.

Momentus XT G2 Momentus XT G1
SATA interface 6Gbps 3Gbps
Spindle speed 7,200 RPM 7,200 RPM
DRAM cache size 32MB 32MB
Flash cache size 8GB 4GB
Available capacities 750GB 500, 320, 250GB
Platter capacity 375GB 250GB
Areal density 541 Gb/in² 394 Gb/in²
Warranty length Five years Five years

We've spent a lot of time talking about the Momentus XT's flash component, but there's a hard drive lurking under the hood, too. The Momentus XT has dual 375GB platters spinning at 7,200 RPM. Each platter has an areal density of 541GB/in², which is a 37% increase over the old model. Higher areal densities can improve performance by putting more data under the drive head with each revolution of the platter, so the new XT should offer a nice step up in performance even when its flash memory is going unused.

The original Momentus XT was available in multiple capacities, but Seagate is limiting the incoming drive to a single 750GB flavor. The old XT's lower capacity points simply didn't sell well, the company says. The original 500GB model will persist alongside the new hybrid, but there's quite a difference in price between them. Just hours before its official product announcement, Seagate changed the price of the 750GB XT from $189 to $245. The old 500GB model currently sells for around $150 online.

Both drives are covered by a five-year warranty, which is considerably longer than the three years of coverage commonly attached to consumer storage products, SSDs included. Only Western Digital's Black series of hard drives matches the XT's five-year warranty.