Ultrabook storage architectures: Hybrid HDD vs cache SSD

June 15, 2012 — Cache solid state drives (SSD) will remain the mainstream ultrabook storage solution, according to IHS, even as hybrid hard disk drives (HDD) offer some consolidated storage advantage. Hybrid HDDs use a built-in layer of NAND flash memory.

Cache SSDs are the leading storage form factor in ultrabooks, growing to 23.9 million units shipped in 2012, a 2,660% increase over 2011.

Figure. Worldwide shipments of storage drives with a solid-state drive component.
SOURCE: IHS iSuppli Research.

 (Millions of units)




Hybrid HDDs




Consumer SSDs




Cache SSDs




Cache SSDs are a discrete, separate memory component alongside the device’s HDD, with both elements housed separately. A sample cache SSD configuration from Acer’s Aspire S3 ultrabook carried a 20GB SSD next to 320GB of hard disk space. Cache SSD shipments will jump to 67.7 million units next year, exceed 100 million in 2015, and hit 163 million by 2016.

The new combination of a traditional HDD and integrated NAND flash layer in one form factor is tailored for ultra-thin computing applications like ultrabooks. The Momentus XT hybrid product from Seagate Technology has up to 8GB of single-level-cell (SLC) NAND and 750GB of memory on two 2.5” platters. Western Digital Corp. and Toshiba Corp. are also considering hybrid HDD products, with drives containing 8GB or more of NAND cache. Hybrid HDDs will see 2.0 million units shipped this year, doubling 2011’s count, then ramp to 25.0 million units by 2016.

Dedicated SSDs, a flash storage architecture that contain no cache component, will hit 18.0 million units shipped in 2012 for consumer applications, gradually ramping up to 69.0 million units in 2016.

Cache SSD came about because dedicated solid state drives were too expensive for PC designs, said Ryan Chien, analyst for memory & storage at IHS. However, a combined physical HDD with a smaller cache component provided faster responsiveness and larger capacities without prohibitive pricing.

Cache SSDs offer more advantages than consolidated hybrid HDDs. Discrete cache SSDs and HDDs are much more scalable and efficient for mainstream storage, with a broad selection of drive manufacturers. SSDs and HDDs have lately been focused on more mobile sizes, requiring few changes 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 7mm 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.

Also read: HDD industry hits reset on prices, markets, technologies post-floods

Learn more in the IHS iSuppli Storage Market Brief report.

IHS (NYSE: IHS) provides information, insight and analytics in critical areas that shape today’s business landscape. For more information, visit www.ihs.com.

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