Seagate SSHDs cache writes in dual-mode NAND

This morning, Seagate announced a new generation of hybrid drives that will target notebooks and desktops alike. We spoke with product manager David Burks this afternoon and have new details to report, including the revelation that the latest version of the Adaptive Memory caching technology has the ability to cache some host writes. The SSHDs use a "tiered writing strategy" that prioritizes data in a similar manner to the read caching algorithms. In order to prevent data loss, cached writes are always written to the mechanical platters. According to Burks, the drives also have enough built-in capacitance to flush their write caches if the power is cut unexpectedly.

Perhaps the most intriguing element of Adaptive Memory's new write cache is how it interacts with the NAND. The flash is configured in a "combo mode" that allows a portion to be addressed as single-level-cell (SLC) memory and the rest to be treated as multi-level-cell (MLC) NAND. Interesting. Only writes and boot data are housed in the SLC portion, Burks says.

This new, dual-mode NAND is apparently faster than the SLC flash used in the old Momentus XT. MLC NAND has lower write endurance, of course, but Seagate's implementation boasts a "200% design margin" for a five-year lifespan. The flash shouldn't burn out under normal client workloads. However, the warranty coverage runs out after three years.

Thanks to their faster flash, the Laptop SSHD and its Thin counterpart are supposed to deliver better performance than the old Momentus XT. That's a particularly impressive feat considering the new models drop the spindle speed of the mechanical platters from 7,200 to 5,400 RPM. Seagate is phasing out 7,200-RPM notebook drives, and its 2.5" hybrids are following suit. Burks says offering only a single 5,400-RPM platform for mobile drives will allow Seagate to focus on its development, and he claims Adaptive Memory largely negates the benefit of faster spindle speeds.

Nevertheless, Seagate's first desktop-bound SSHDs will spin their platters at 7,200 RPM. The 1TB and 2TB models both use terabyte platters, and they'll eventually be joined by a higher-capacity variant. That drive will have a slower spindle speed, but it will purportedly deliver near-identical performance to the 7,200-RPM models.

All of the new SSHD models have 8GB of flash. That might seem a little small, but Seagate insists there's enough NAND capacity to cache data effectively for typical consumer and commercial workloads. The Adaptive Memory algorithms only cache data that will be substantially faster to read from the flash, so they're somewhat more selective than software-based solutions like Intel's Smart Response Technology.

Around the middle of the year, a new addition to the mobile SSHD family will target ultrabooks and provide compatibility with Intel's caching scheme. This drive will feature a larger NAND cache controlled by Smart Response Technology. Seagate has been working with Intel, Western Digital, and the SATA-IO body to develop an extension to the Serial ATA spec that facilitates such hybrid configurations. I'm curious to see how the SRT-managed model stacks up against the standard SSHDs.

The 2.5" SSHDs announced this morning are slated to hit vendors like Newegg within a week, and they should be pretty affordable. Expect to pay about $79 for the 500GB model and $99 for the 1TB variant. When the 3.5" desktop version becomes available at the end of April, the 1TB flavor will ring in at around $99, and its 2TB sibling will be priced at $149. Seagate is shooting for a $15-20 premium over purely mechanical alternatives. Calculate the cost per gigabyte, and you're looking at huge savings versus solid-state drives.

A pair of SSHDs should be in our labs soon, so we'll be able to take a closer look at their performance and potential value proposition. It's important to note that Seagate doesn't see these hybrids as replacements for traditional hard drives, which will continue to appeal to budget-conscious users, or for SSDs, which have their place for performance-minded folks like enthusiasts. SSHDs are designed to address the market between those extremes, and the concept certainly has some appeal, particularly in notebooks with only a single drive bay.

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