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Hybrid hard drives: How they work and why they matter

Jon L. Jacobi | Oct. 22, 2012
An SSD can read and write data many times faster than the best mechanical hard drive. On the downside, flash memory is many times more expensive than the innards of a typical hard drive, so manufacturers have limited their SSD capacities to hit reasonable price points: A 128GB SSD costs about $130, and for that same price tag, you can buy a 3.5-inch desktop hard drive that delivers 2TB of storage, or a 2.5-inch laptop drive that provides 1TB of storage.

An SSD can read and write data many times faster than the best mechanical hard drive. On the downside, flash memory is many times more expensive than the innards of a typical hard drive, so manufacturers have limited their SSD capacities to hit reasonable price points: A 128GB SSD costs about $130, and for that same price tag, you can buy a 3.5-inch desktop hard drive that delivers 2TB of storage, or a 2.5-inch laptop drive that provides 1TB of storage.

Two years ago, Seagate (quickly followed by Samsung) introduced a drive that married a small SSD with a mechanical drive. The objective was to deliver the superior speed of an expensive SSD, while retaining the higher capacity and lower cost of a conventional hard drive. Now that Toshiba and Western Digital are joining the party, it's a great time to explain in more detail what hybrid drives are and how they operate.

Hybrid drives work much in the same way as the current dual-technology configurations in many gaming and power-user PCs, as well as some ultraportable laptops. Such systems have a small, discrete SSD to hold the operating system and frequently used data, augmented by a more capacious conventional hard drive for less frequently accessed data and large collections of documents and digital media.

Current hybrid drive designs, in contrast, deliver both technologies within a single physical unit, and they employ software caching algorithms (rather than relying on the user's brain) to decide which data belongs on the SSD portion and what goes on the drives platters.

These caching algorithms reside in the hybrid drive's firmware, not the device driver. To the computers operating system, a hybrid drive appears as a single unit with the SSD portion acting strictly as a large cache. The cache is nonvolatile, so the data doesn't disappear when power is absent.

You can find several hybrid designs on the market, but the most common is a 2.5-inch version meant for laptops, Seagate's Momentus XT SSHD. Seagate refers to the caching logic it uses on the Momentus XT drives as Adaptive Memory technology. The thinner 7mm, 2.5-inch drives that Toshiba and Western Digital recently announced are destined for Ultrabooks. They will likely use similar technologies with similarly suitable names, although either company might opt to skip caching and produce a dual SSD/hard drive in a single physical package. In any case, a caching algorithm will track the files you load the most often (operating system files, applications, and the like), and store them on the SSD portion of the drive. From that point on, these files will load into memory much faster than they did from the mechanical drive, although some overhead will be involved as the computer determines whether the file in question resides on the SSD. No caching will have occurred the first time you use a hybrid drive, so its initial performance will be the same as that of a mechanical hard drive, but the speed will increase over time.

 

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