If you have a cursory understanding of storage technologies, odds are you know that hard disk drives and solid state drives are not the same. While we can spend hours scrutinizing and debating the distinctions between a pair of SSDs, nearly everyone knows the differences between HDDs and SSDs in broad strokes. Hard drives are still the best source of cheap gigabytes, but they’re undeniably slower than solid-state drives. Those fast SSDs continue picking up speed, but it can be hard to justify the cost, especially when compared to an HDD.
If you have a cursory understanding of storage technologies, odds are you know that hard disk drives and solid state drives are not the same. While we can spend hours scrutinizing and debating the distinctions between a pair of SSDs, nearly everyone knows the differences between HDDs and SSDs in broad strokes. Hard drives are still the best source of cheap gigabytes, but they’re undeniably slower than solid-state drives. Those fast SSDs continue picking up speed, but it can be hard to justify the cost, especially when compared to an HDD.
What happens when you cross a HDD with a SSD? No, this isn’t a setup for the nerdiest, storage-related joke. Solid-state hybrid drives (SSHDs) occupy the middle ground between hard drives and solid-state drives. They take a little from column A and a little from column B in an attempt to be an intriguing compromise candidate. But as storage has evolved to a two-party system, do hybrid drives have a compelling enough platform to warrant attention?
Caching In
“Solid state” are the two words that sell a SSHD, because of the underlying premise that solid-state storage leaves magnetic storage in the dust on the test track. Who wouldn’t want a gargantuan hard drive with the ability to read and write data at the speed of a SSD? A product like that practically sells itself.
Here’s the rub: Hybrid drives aren’t able to ferry around all of their data at speeds that strain the SATA bus (like modern SSDs can). The vast majority of a hybrid drive’s capacity consists of the same platters found in traditional hard drives, and no amount of technological fairy dust will make them inordinately faster. However, although hybrid drives can’t break the rules, they’re engineered to bend the hell of out of them.
Bending the rules begins with a handful of NAND Flash memory incorporated into the hybrid drive. The amount of NAND differs from hybrid drive to hybrid drive, typically ranging between 8GB and 16GB, but it’s always a sliver of the drive’s overall capacity. With a token amount of NAND, conventional solid-state tactics of depositing an operating system in its entirety and adding a handful of frequently used applications to keep it company don’t apply. Manufacturers like Seagate, which pioneered hybrid drives as we know them today, instead developed ways a handful of on-drive NAND could work hand-in-hand with magnetic storage to deliver better performance than traditional HDDs.
Rather than storing data indiscriminately, a hybrid drive’s NAND serves a cache for frequently accessed data. Conceptually, it functions similarly to a browser cache or prefetch folder, which keeps high-priority data in a location where it can be rapidly accessed. On a hybrid drive, rather than storing programs in NAND in their entirety, the hybrid drive’s own controller determines which application files—including the OS—benefit the most from residing in NAND. The hybrid drive controller’s authority to determine which files are stored where is key to understanding why packaging a NAND cache together with hard disks is effective. In essence, the hybrid drive’s controller is better than the host system’s OS and drivers in knowing how to use the drive’s NAND to increase performance. Further, because the data exchange between NAND and magnetic storage media occurs within a single drive, as opposed to a logical hybrid drive of a separate flash cache and HDD, a hybrid drive doesn’t consume as many system resources when it caches data.
As far as throughput is concerned, a hybrid drive can’t always match a SSD. Otherwise, what would be the point of using discrete SSDs? But compared to traditional hard drives, hybrid drives significantly diminish the importance of conventional performance metrics. Typical hard drives achieve performance gains by increasing the speed at which the read/write head scoops data from the platter. Cramming bits closer and closer together on the platter (increasing areal density) helps; so too does ramping up the hard drive’s spindle speed. Basic logic dictates that higher areal density and RPM ratings result in superior throughput, all things being equal.
Hybrid drives’ NAND cache turns the fundamentals of hard drive performance on its head. Cached data bypasses magnetic storage and escapes the media’s performance limitations. In this situation, the NAND’s speed is a big factor. In fact, when a hybrid drive leverages its NAND, firmware, and controller, the drive’s spindle speed takes a back seat: Depending on workload, newer generations of hybrid drives with a rotational speed of 5,400rpm can outshine older hybrid drives whose platters spin at 7,200rpm.
Enthusiasts with a solid understanding of SSDs know that firmware frequently plays the role of unsung hero. When two hybrid drives have similar anatomies, firmware is the genetic engineering, the remixed DNA that makes one hybrid drive better than another. Firmware matters.
More Than Hardware
To use yet another analogy, in any storage drive, the controller functions like a traffic cop, helping direct data between destinations in the most efficient manner possible. Take a look at the job description for a hybrid drive’s traffic cop. It reads, “Frequently accessed data needs to travel across the NAND cache interstate. Direct all other data to Platter Street.”
Hybrid drive manufacturers are responsible for writing the best possible training manual for their traffic cop. That’s firmware, the place where drive makers can inject proprietary code and algorithms. One such example is Seagate’s Adaptive Memory Technology.
Adaptive Memory Technology consists of algorithms engineering to be self-learning. The algorithms identify hot data (which is data that offers the most significant host-level performance boost when stored in NAND) and instruct the drive’s controller to shift this data from magnetic storage to NAND Flash. Adaptive Memory Technology also operates independently from the host system and storage drivers. Put another way, Seagate understands better than anyone or anything else how to best take advantage of their hybrid drives’ NAND, and they bake that understanding into Adaptive Memory Technology.
SSHDs At Work
As a general rule of thumb, if you can afford to store data on a SSD, you should. On the other hand, how much data does someone really need in NAND to set up their storage subsystem for ideal performance? Ultimately, not as much as you might think. Seagate believes their SSHDs’ 8GB MLC NAND cache is plenty for the files associated with booting, restarting, application launching, and so on.
Like most technologies, exceptions exist. Hybrid drives fare poorly with applications that use large and/or regularly changing data sets. CAD programs and HD video editing software are two examples. Most other apps stand to benefit greatly from the mere presence of an 8GB NAND allotment.
More broadly, SSHDs thrive in certain scenarios. Hybrid drives are available in 3.5-inch and 2.5-inch form factors; the latter size is particularly well-suited for laptop users who require an infusion of speed but also need lots of storage at a price point closer to traditional HDDs. SSHDs fill the role perfectly, especially if a given laptop is limited to a single drive bay. Desktop users who insist on peak performance should also consider SSHDs for their mass storage needs. Since platter-based storage remains a vital element of most systems, SSHDs’ complement of MLC NAND gives them a distinct advantage over standard hard drives.
Storage enthusiasts and the tech press face the temptation of considering solid-state drives and hard drives in adversarial roles. Not only can the two types of storage drives coexist, they can also cooperate. Hybrid drives can do things SSDs and HDDs can’t, and that makes them an intriguing option.
Seagate Laptop SSHD 1TB Review
Seagate Desktop SSHD 4TB Review
Seagate Enterprise Turbo SSHD Review
Seagate Desktop SSHD Review
Seagate SSHD Thin Review
Sign up for the StorageReview newsletter
