Already the leading storage form factor in ultrabooks, cache SSDs will see their shipments rise even more this year to 23.9 million units, up by an astounding 2,660 percent from just 864,000 units in 2011, as shown in the figure below. Shipments will then jump to 67.7 million units next year, cross the hundred-million-unit mark in 2015, and hit 163 million units by 2016.
In comparison, shipments of hybrid HDDs will reach 2.0 million units this year, up from 1.0 million units in 2011, on their way to reaching 25.0 million units by 2016. A third form of flash storage—dedicated SSDs that contain no cache component—will attain shipments this year of 18.0 million in consumer applications, gradually ramping up to 69.0 million units in 2016.
Consisting of a traditional hard disk drive and an integrated NAND flash layer within one self-contained form factor, hybrid HDDs are a new storage option being considered for the super-thin ultrabooks.
The Momentus XT hybrid product from Seagate Technology has up to 8 gigabytes (GB) of single-level-cell NAND and 750GB of memory on two 2.5-inch platters. Seagate rivals Western Digital Corp. and Toshiba Corp. are also considering hybrid HDDs of their own, with drives containing 8GB or more of NAND cache.
In contrast to hybrid HDDs, cache SSDs are employed as a discrete, separate memory component alongside a hard disk drive, with both elements existing side by side, not together in one housing unit. A sample cache SSD configuration from Acer’s Aspire S3 ultrabook carried a 20GB SSD next to 320GB of hard disk space.
“The cache SSD solution was first hit upon by PC manufacturers because the use of a dedicated solid state drive proved too expensive when passed on to consumers in the retail market,” said Ryan Chien, analyst for memory & storage at IHS. “However, a combined physical hard disk drive with a smaller cache component allowed PC makers to reap the advantages of faster responsiveness and larger capacities while keeping costs down.”
Cache SSDs also offer more advantages than consolidated hybrid HDDs. For instance, discrete cache SSDs and hard disk drives are much more scalable and efficient for mainstream storage, given the broad selection of drive manufacturers. And because SSDs and HDDs have lately been focused on more mobile sizes, few changes are needed for cache SSDs or thin HDDs to keep their manufacturing processes cost effective.
Moreover, the expected evolution of cache SSDs to a swappable mSATA form factor not only helps narrow the convenience advantage currently enjoyed by hybrid HDDs but also facilitates upgradability akin to DRAM modules or USB drives. HDDs sized 7 millimeters (mm) are available with 500GB in 2.5-inch platters, with a 5-mm z-height as the next step, while hybrid HDDs are still 9-mm high. SATA solid state drives are also getting denser, and NAND on the motherboard is becoming more feasible.
Such benefits overall highlight the strength of discrete, dedicated hardware emblematic of cache SSDs over those of hybrids, which tend to make compromises in exchange for volume implementation.
Cost concerns, longer design cycles and tighter engineering tolerances in the case of hybrid HDDs also add to their difficulty of use in ultrabooks.
As a result, the gain achieved through a consolidated form factor supposed to be a strength of hybrid HDDs will actually be a weakness in a few years as cache SSDs overcome that advantage. For their part, cache SSDs will remain as the favored storage solution in ultrabooks, with penetration of the desktop and the rest of the notebook segments anticipated to follow.