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Seagate's Laptop Thin SSHD 500GB hybrid drive reviewed

Introducing the drop-top Prius

What do the McLaren P1, the Porsche 918, and the Ferrari LaFerrari all have in common? They're limited-edition supercars with price tags well out of reach for mere mortals. Also, they're all hybrids. Even in the world of high-end exotics, internal combustion engines are being paired with electric motors.

Right now, hybrids seem likely to be the next evolutionary step for automobiles. Their electric motors are both efficient and environmentally friendly, while their fuel-based engines provide the road-trip range people have come to expect. There's a similar parallel in the PC storage industry.

Modern SSDs are much faster than traditional hard drives, but even with Moore's Law chipping away at NAND prices, they remain relatively expensive if you want loads of storage. SSDs in the 240-256GB range typically cost around $200, and they still don't give you a lot of space, especially if your media library is going to share the drive with your OS, applications, games, and other data.

While mechanical hard drives can't keep up with the performance of SSDs, their spinning platters can store a lot more data per dollar. Marrying the two in a hybrid configuration is a no-brainer, and it's incredibly easy to do in desktop systems that can accept multiple drives. Notebooks, however, are a whole other animal. Some laptops permit mechanical drives to ride shotgun alongside mini mSATA SSDs, but most are limited to a single drive.

Since mid-2010, Seagate's solution for single-drive notebooks has been the Momentus XT, a hybrid that combines flash memory and mechanical platters in one 2.5" chassis. The original model and its second-generation successor have had some appeal, but they've been hampered by read-only flash caches that ignore incoming writes from the host. That limitation has been lifted in Seagate's latest hybrids, which can cache both read and write requests.

The first of this new generation to hit our labs is the Laptop Thin SSHD 500GB. (SSHD stands for solid-state hybrid drive, in case you're wondering.) Although painfully generic, the model name gets the point across. This is a laptop drive with a thin form factor and 500GB of storage. And it's eminently affordable, with a street price hovering around 80 bucks. Naturally, we had to take a closer look.

Thin is in
Most 2.5" hard drives are 9.5 mm thick, but the Laptop Thin SSHD squeezes its guts inside a 7-mm case. This thinner chassis is a better fit for the growing field of super-slim notebooks spawned by Intel's ultrabook initiative. It also represents a first for Seagate's hybrids, which have previously been limited to 9.5-mm models.

The 9.5-mm Momentus XT (left) next to its Laptop Thin sibling (right)

To help its hybrid diet down to the smaller form factor, Seagate removed one of the platters. The Thin Laptop SSHD has a single disc with a 500GB capacity. Bits are packed with an areal density of 705Gb/in², which allows each side of the platter to store 250GB. That bit density is 30% higher than that of the old Momentus XT 750GB, whose dual platters pack 541Gb/in².

An increase in areal density usually leads to better performance for sequential transfers. The more bits per square inch, the more data passes under the drive head with each revolution of the platter. On the Laptop Thin SSHD, however, there is one rather significant catch. The platter spins at only 5,400 RPM—25% slower than the Momentus XT's 7,200-RPM spindle speed. This slower rotational speed negates much of the advantage of the increased bit density.

Seagate contends that spindle speed is less important for hybrid drives, and the benchmark results later in this review will shed some light on that claim. The firm is certainly confident, because it's committed to stop making 7,200-RPM notebook drives altogether. 5,400-RPM hybrids will replace high-speed drives in Seagate's notebook lineup, allowing the company to focus development on a single mechanical platform for mobile PCs. That platform will presumably be shared by SSHDs, notebook hard drives, and external storage products.

We tend to recommend 5,400-RPM drives only when they're being paired with SSDs, which is essentially what's happening here. In addition to 500GB of mechanical storage, the Laptop Thin SSHD has 8GB of flash memory. Like in previous Seagate hybrids, the flash is governed by a caching mechanism dubbed Adaptive Memory. This scheme categorizes data based on two criteria: how frequently it's accessed and whether relocating it to the flash will improve performance. Solid-state storage is orders of magnitude faster than mechanical storage for random accesses, but the two are more evenly matched when data is accessed sequentially.

Adaptive Memory can see precisely how data is organized on the disk, a level of insight absent from software-based caching mechanisms like Intel's Smart Response Technology. According to Seagate, this low-level visibility allows its hybrids to make better decisions about what to put in the flash. Because the caching magic works entirely within the drive, the Laptop Thin SSHD requires no drivers or software. It'll work with any operating system and hardware platform.

Folks familiar with hybrid storage will note than the Laptop Thin SSHD's 8GB flash cache is relatively small. The NAND footprint hasn't grown since the last Momentus XT, which is a particular concern given the new model's ability to cache data for writes in addition to reads. Product manager David Burks told us Seagate experimented with different cache sizes and found that 8GB was big enough for mass-market workloads. The data access profiles of typical consumer and commercial workloads don't change dramatically, he said.

Seagate hasn't revealed many details on exactly how the write caching mechanism works, but Burks confirmed that data is prioritized in much the same manner that it is for reads. Whether an incoming write can be processed faster by the flash plays a role in determining whether it will be cached there. We haven't heard back from Seagate on how much of the flash is reserved for incoming writes and whether the distribution is static or dynamic. Burks did, however, tell us that the drive has enough onboard capacitance to ensure that the contents of its write cache can be written to the disk in the event of unexpected power loss.