SSHD is the latest buzzword in the PC storage industry. Although the branding is fresh, the term refers to a class of products that's been around for quite some time. Otherwise known as solid-state hybrid drives, SSHDs combine mechanical platters with flash-based cache memory. They first popped up in the Windows Vista era and were designed to work with that operating system's ReadyDrive feature. Vista wasn't exactly popular at the time, and hybrids faded into obscurity.
Seagate revisited the idea in 2011 with the Momentus XT notebook drive. This hybrid solution was independent of the OS, instead relying on drive-level intelligence to manage the flash. All things considered, it was a pretty good compromise for notebooks. A second generation Momentus XT hybrid followed the initial model, and the third iteration of the platform debuted with the SSHD moniker earlier this year.
When the latest batch of notebook-bound SSHDs was introduced, Seagate revealed that a desktop version would soon join the family. This simply named Desktop SSHD would discard the 2.5" form factor of previous hybrids in favor of a larger 3.5" body packed with 2TB of 7,200-RPM mechanical storage. Naturally, we were intrigued. We've been bugging Seagate about the drive ever since, and we finally got our hands on one to review. Let's see what the world's first hybrid desktop drive can do.
Porting Adaptive Memory to the desktop
Like its notebook predecessors, the Desktop SSHD adds a small amount of flash memory to a traditional hard drive. The 8GB of onboard NAND matches the flash capacity of the current generation of notebook SSHDs. 8GB doesn't sound like a lot, but Seagate contends that typical consumer and commercial workloads deal with relatively small data sets. The firm says it experimented with different cache sizes and found 8GB to be sufficient for most workloads. Seagate concedes that its SSHDs aren't equipped to accelerate workloads with larger data sets, such as HD video editing and CAD work. The Desktop SSHD is designed more for everyday folks than for power users.
The onboard flash can hold 8GB in an MLC configuration with two bits per cell. However, the SSHD addresses a portion of the flash as one-bit SLC NAND, which has half the effective storage density. Seagate has repeatedly dodged our questions about exactly how much of the flash is addressed as SLC NAND and whether the SLC-to-MLC ratio is static or dynamic. We do know that the MLC NAND is reserved for caching data for reads, while the SLC zones store boot data and cache some incoming writes.
There are a couple of reasons to split caching duties between SLC and MLC NAND. Due to the verification steps required when programming flash memory, one-bit cells write much faster than their two-bit counterparts. They typically have ten times the endurance, as well. Those characteristics make SLC memory ideal for write caching. Meanwhile, MLC's higher density allows more data to be speculatively cached for read requests. That part of the flash will receive plenty of read requests, but it will only be written when Adaptive Memory copies over frequently accessed data from the platters.
Situated on the left in the picture above, the Toshiba NAND is built on a 24-nm fab process. It should be robust enough for the long haul; Seagate says the Desktop SSD has a 200% endurance "design margin" based on five years of typical use.
There's only one flash package, so it's important to establish performance expectations early. Modern SSDs extract much of their speed from massive parallelism. Current controllers are at their best when addressing at least 32 NAND dies across eight memory channels. Lower-capacity SSDs can't saturate all those parallel pathways, which is why they typically carry slower performance ratings.
The Desktop SSHD's flash component is effectively a really small SSD. Toshiba's part number decoder indicates that the NAND package has only two channels, while Seagate's spec sheet pegs the flash's average data rate at 190MB/s—only a little better than one third the average throughput of today's fastest consumer SSDs. As far as sustained speeds are concerned, the flash isn't that much faster than the Desktop SSHD's mechanical component, which is rated for 158MB/s. Seagate doesn't quote the random access time of the flash, which should be orders of magnitude quicker than the 12-millisecond response time of the platters.
Deciding which data to cache in the flash falls upon Seagate's Adaptive Memory tech. These firmware algorithms categorize data based on how frequently it's accessed and whether doing so from the flash would improve performance. This approach draws from low-level information about how data is laid out on the drive—intelligence that software-based caching solutions lack—and it's completely transparent to the operating system. You don't even have to install a driver.
Seagate says the Desktop SSHD's Adaptive Memory algorithms are exactly the same as those used in the Laptop Thin SSHD. The skinny notebook drive has a much slower 5,400-RPM mechanical component, though. Adaptive Memory doesn't treat the two mechanical components differently. Instead, it focuses on determining whether data is sequential or random. Sequential data stays on the platters, leaving more of the flash available for random I/O.
Intercepting writes is a relatively new addition to the SSHD's job description; until this latest generation, Seagate's caching scheme was limited to storing data for read requests. Caching incoming data comes with some inherent risk, but Seagate says all writes are eventually passed along to the mechanical platters. If the drive loses power unexpectedly, onboard capacitors provide enough juice to flush the write cache.
|Spindle speed||7,200 RPM|
|Average solid-state data rate||190MB/s|
|Average mechanical data rate||158MB/s|
|Average read/write seek time||< 12 ms|
|Idle power||< 3.9W|
|Warranty length||Three years|
The Desktop SSHD spins dual 1TB discs at 7,200 RPM. The platters boast an areal density of 625 Gb/in², and they're identical to the ones used in Seagate's current lineup of traditional desktop drives. Like those models, the hybrid has 64MB of DRAM cache and a 6Gbps Serial ATA interface.
Seagate offers two Desktop SSHD models right now. The 2TB variant we're looking at today sells for $140, while the 1TB version is priced at $100. (The 1TB version is basically the same, just with one platter instead of two.) Those prices are $30-40 higher than the going rates for equivalent 7,200-RPM hard drives in Seagate's stable.
The price premium at least comes with a warranty bonus. Unlike Seagate's standard desktop drives, which have two-year warranties, the SSHDs are covered for three years. That coverage matches the warranty period of most consumer-grade SSDs.
With its circuit board screwed back into place, the Desktop SSHD doesn't look that different from standard hard drives. We know better, and now it's time to see if the flash cache pays off...
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