Will the HDD market be swept by hybrid hard drives?
09/01/2007
Suddenly PC purchasers have four choices when buying mass storage for their PC: standard hard-disk drive (HDD), a hybrid HDD, a solid-state drive (SSD), and Intel’s new Turbo Memory, combined with either. Why all the choices? What does each bring to the party?
 Figure 1. NAND and DRAM price per gigabyte history. (Source: Objective Analysis, September 2007) |
The three newer technologies all use NAND flash. NAND has undergone a significant price decline over its short history, driven by the process shrinks common to all semiconductor devices, coupled with the accelerating phenomena of multilevel cells (MLC) and a conversion to 300mm wafers. The confluence of these has allowed NAND’s average price per gigabyte to zoom right past that of DRAM in 2004 to become the cheapest of all semiconductor memories (see Fig. 1). PC architects have noticed this, and are looking for ways to use NAND to improve PC performance at a reasonable price.
The key feature driving the adoption of NAND in PCs is the SuperFetch technology that Microsoft introduced with its new Windows Vista operating system. This technology allows the software to monitor HDD usage patterns and to preload data that may be useful soon. SuperFetch profiles the PC user’s typical behavior, and can preload code that is frequently needed at boot-up into a NAND if there is one in the system. This NAND can be on a USB flash drive, in the HDD, or on the PC’s motherboard.
While thinking in this vein, Microsoft’s developers considered other ways to use NAND in the PC, and came up with the idea of storing multiple HDD writes onto the NAND, gathering them together into a single large write that would happen infrequently. This infrequent use allows the HDD to remain spun down significantly more often, reducing power consumption, noise, and heat dissipation, while improving mechanical reliability and even access times. To the user, this should mean faster operation, longer battery life, and a more reliable system.
Hybrid hard-disk drives
To embody all these ideas, Microsoft worked with HDD makers to develop the hybrid hard drive (HHD) (also called the hybrid hard-disk drive or H-HDD). The HHD is simply a standard HDD with the addition of a NAND write cache. This does not replace the DRAM cache built into all HDDs since the 1990s. The DRAM cache stays because of its faster write and lower power than NAND. DRAM is used to accelerate reads, and NAND is used to gather writes, and to support SuperFetch.
How does this work? In essence, the NAND is a part of a cache that sits between the HDD and the PC’s DRAM main memory. The cache does what all caches do: it makes the HDD appear faster to the DRAM. The DRAM cache makes reads faster, and the NAND cache makes writes faster. The memory hierarchy of both today’s systems and the new systems with a NAND cache are shown in Fig. 2.
 Figure 2. Memory hierarchy: without NAND HDD cache (left) and with it (right). (Source: Objective Analysis, September 2007) |
Like any cache, the NAND and DRAM don’t have to be internal to the HDD. Intel recognized this and developed its Robson technology (recently renamed “Turbo Memory”). This technology operates similarly to the NAND in a hybrid drive but locates it on a PCI Express board rather than within the hard drive. Intel contends that this is a superior approach, since the NAND size can be upgraded at a modest cost without discarding the HDD. Intel also points out the inventory management advantages of stocking several inexpensive densities of PCI cards and only one capacity of HDD instead of stocking several HDDs with different cache sizes. This argument would make more sense if the $50 price of a 1GB Robson board were closer to the $7 cost of a gigabyte of NAND than to the $50 average price of a 2.5 in. HDD. Although a Turbo Memory could be coupled with a hybrid drive or an SSD, there would be little advantage to doing so. The NAND in the Turbo Memory would simply replicate the NAND in the hybrid drive or that in an SSD.
Solid-state drives
What about SSDs? These are today’s hot item. Why is that?
Although SSDs have been around for a long time-Texas Memory Systems has been shipping them since the 1970s-they are only just starting to attract mainstream attention. Part of this is because Samsung has recently taken a strong stand in advocating SSDs for the PC.
Since SSDs are 100% NAND or DRAM, they can already take advantage of the features of Microsoft’s ReadyDrive. Since DRAM drives will not be able to boot quickly from a powered-down state, they are unlikely to be used in a PC application. These high-ticket devices are always sold in very high capacity models aimed at supercomputing applications.
Solid-state drives are, and always have been, significantly more costly than their HDD counterparts. Early this year, SanDisk introduced a 32GB SSD, which the company plans to sell for $350 in volume. Compare this to the $100 price for a 120GB 2.5 in. HDD. Not only is the HDD less than a third of the price, it also offers nearly 4× the storage, for a cost per gigabyte that is one thirteenth that of the SSD. The SSD needs some very compelling features to overcome this price/capacity differential.
SSDs offer faster operation and lower power (for most models), less weight, and ruggedness. Applications for SDDs are found in military or industrial areas, as well as in some financial and medical fields.
Note that the hybrid drive is able to address a number of these issues. Because it keeps the heads parked for a far larger proportion of the time, hybrid drives are much more rugged and durable than a standard HDD. The NAND flash, by satisfying data requirements for the bulk of the disk activity, also helps to reduce power and increase the apparent speed of the HDD for a large percentage of disk accesses. Since it’s a cache, this is a statistical number, and is likely to be over 90% for most applications. This means that for a price relatively close to the $100 of the HDD, users will realize over 90% of the benefits of an SSD.
Most users (those who don’t absolutely require the ruggedness of an SSD) will choose a less expensive hybrid over a more expensive SSD unless they lose sight of the hybrid drive’s similarity in performance and durability. It is very likely that the high price differential will focus so much attention on this area that they will instead be keenly aware of the advantages of the hybrid drive.
Will SSDs always be more expensive than HDDs? NAND prices have been falling at an unheralded rate lately. Isn’t it likely that NAND will eventually catch up with HDD and become a cheaper medium?
As shown in Fig. 1, NAND has sped past DRAM, seeing price drops unprecedented in semiconductor memory. As the exercise above showed, though, there is an extremely wide gap between HDDs and NAND from the standpoint of price per gigabyte. HDDs’ price per gigabyte historically has declined at a rate of about 50%/year, making it a moving target. Even if this were not the case, and HDDs’ price per gigabyte suddenly went flat, it would take NAND until 2011 to match HDDs’ current price per gigabyte of 83 cents. In reality, HDD’s 50% annual price per gigabyte decline is very close to that of NAND, which had a decline averaging 50.25% over the period charted in Fig. 1. It is quite possible that HDDs will always stay far ahead of NAND.
To boil this down to a single sentence: SSDs will have a very hard time competing against HDDs because of their price difference, and the advent of the hybrid drive-with its improved ruggedness, power consumption, and performance-will make it even harder.
With this in mind, Objective Analysis predicts that hybrid hard drives will sweep the HDD market very shortly following their widespread availability. This availability should begin toward the end of this year. We anticipate that either hybrid drives, or the alternative Turbo Memory technology, is likely to be found in all new PCs sold by the end of 2009. Like the DRAM cache found in every HDD today, this feature will be nearly invisible to the average PC purchaser, but will be seen by OEMs as a minimum requirement to compete in the PC marketplace.
Jim Handy is the director of Objective Analysis, an independent market research firm. Objective Analysis, PO Box 440, Los Gatos, CA 95031-0440 USA; ph 408/356-2549, e-mail Jim.Handy @ Objective-Analysis.com.