The rapid rise in popularity of mobile audio devices like the Apple iPod is bringing a renewed focus to the two main forms of nonvolatile storage: NAND flash and hard-disk drives. Traditionally, hard drives were the primary mass-storage medium, though recent growth in the NAND market has been extraordinary. The density increases seen in NAND are fueling speculation that it will soon displace hard drives as the dominant technology for data storage. Hard drives, however, are well-entrenched in many markets and have significant advantages over NAND flash devices. This suggests that any large-scale migration to NAND is unlikely and maybe unnecessary. In the longer term it is unclear which technology will dominate, but in the short term hard drives and NAND flash will compete within certain market segments.
NAND flash devices are solid-state memories with no moving parts, relying instead on storing small amounts of electrical charge in semiconductor transistor structures. Hard drives, on the other hand, rely on a mechanical system consisting of a spinning platter that retains information in magnetic storage elements. Both NAND and hard drives have experienced phenomenal density growth, with hard drives doubling their storage capacity every two years and NAND flash devices doubling their density every year.
The doubling of NAND flash density each year is a remarkable technological achievement, and current road maps predict that this doubling will continue for at least the next three years. One reason this doubling is so remarkable: It exceeds the rate of Moore's Law, which projects that a chip's transistor density will double about every 18 months. The other remarkable item about the growth in NAND flash is that it has replaced DRAM as the technology driver for several semiconductor companies. This means that NAND flash is produced in the most advanced semiconductor process nodes ahead of any other device. For example, the first device produced in Samsung's 73-nanometer process was a 4-Gbit single-level-cell NAND flash. Toshiba is reportedly produc- ing a 70-nm 8-Gbit multilevel-cell NAND flash.
Hard drives have scored remarkable technological achievements and can create large storage elements in a relatively small footprint. A 3.5-inch drive with a double disk, for instance, stores up to 320 Gbytes. The availability of hard drives in smaller form factors of 1 inch, 1.8 inches and 2.5 inches has enabled the development of smaller, advanced consumer products, such as the Apple iPod audio players. The iPod Mini used a 1-inch hard drive, paving the way for the iPod Nano, while the iPod Video uses the 1.8-inch hard drive.
Large capacity and low cost per bit are the principal advantages of hard-drive technology. That makes hard drives an economical storage element for applications that require a large amount of storage. Today's NAND technology cannot match the hard drive's ability to deliver a low cost per bit for large densities. NAND devices, by contrast, are smaller, more robust, require less power and have some speed and performance advantages over disks. NAND flash storage elements are generally smaller than their hard-drive counterparts; this is best exemplified by the size differences between the iPod Nano and iPod Mini. These two MP3 players have comparable densities, but the Nano comes in a smaller form factor and has a longer battery life.
The iPod Mini, iPod Nano and an IRiver player all contain 256 Mbits of SDRAM, though the iPod shuffle does not have a standalone DRAM buffer. Such a buffer is loaded with content; the audio circuitry accesses the DRAM to play this content, since DRAM has faster write and read times. In a buffered hard-drive player, the drive is inactive about 97 percent of the time. When active it's filling the DRAM buffer, so power consumption for a device with a hard drive is not as bad as one might first think.
Apple's iPod Nano is one arena in which NAND flash's small size and data density have beaten hard-disk drives.
Hard drives generally have slower throughput speeds than NAND flash but this does not represent a significant disadvantage, since the DRAM buffer and audio speed requirements make this transparent to the user for audio applications. NAND flash offers much lower latency times and much greater I/O operations per second. This advantage is not as valuable in applications that deal with large file sizes, specifically audio applications, but it does offer advantages in code applications where address instructions are not sequential.
The cost to make both hard drives and NAND devices varies, depending on the amount of storage required. Cost is important and it often drives the type of storage that is selected for a specific application. Hard drives have a much lower cost/megabit of information at almost any density, though the advantage varies depending on the size of the drive. A 1-inch drive, for example, has a lower cost/Mbit rating than a 3.5-inch drive. The reason for the difference in price competitiveness is that there is a base cost to produce a hard drive. The base cost is due to the casing and motor, but once this is in place, increasing the size of the platter brings significant density increases at little incremental cost.
The cost for a NAND storage element is based on the cost to manufacture a silicon wafer and dice it. The cost per die for NAND flash is lower than the cost to manufacture any hard drive, and gives NAND flash producers the ability to capture applications that need lower amounts of nonvolatile memory. But as the amount of memory required grows, the cost of the NAND solution grows in a stepwise fashion, based on the unit size of the NAND flash die. The cost of a large NAND flash storage element quickly exceeds the cost of a comparable hard-drive solution. The cost-parity crossover point is currently estimated to be about 2 Gbytes. NAND flash vendors are eager to move their devices to more-advanced process geometries to decrease the cost per die and increase the density at the cost-parity point.
At the cost-parity point, NAND and hard drives become comparable in terms of density and cost, and it's often where the two technologies compete for applications.
Future NAND road maps continue to show NAND flash density doubling every year, currently a greater pace than the density increases projected for hard drives.
Hard drives or NAND devices are used in a range of applications. Most apps use one of the technologies almost exclusively, since it offers significant advantages. But in a few applications, hard drives and NAND devices compete for storage.
Hard drives are ideal for such applications as personal computers and video recorders, which need lots of storage at low cost. NAND-based drives are unlikely to gain significant design traction in these applications anytime soon, though NAND companies like Samsung have announced 32-Gbyte NAND-based drives. These large NAND drives are fairly expensive relative to hard-drive solutions and thus will likely be used in niche or high-end applications.
But in some applications, the technologies complement one another. Laptops could use a combined NAND/hard-drive storage element, with the NAND flash acting as a type of cache that allowed the hard drive to power down more frequently to save power and increase reliability. The NAND element could also store the standby operating system so the device could come out of standby mode faster.
NAND flash devices have helped develop the digital camera market and, though high-performance cameras use hard drives, it is unlikely those drives will make any significant inroads into this market. Cell phones have also been the domain of flash, but traditionally a different type, NOR flash. Cell phone makers are used to working with flash and, since size is important in this application, it is likely that the manufacturers will use NAND flash to address large-scale storage needs.
Media memory
The mobile media player is the most interesting application for hard drives and NAND flash, since it uses both storage elements. In most cases, NAND flash dominates the MP3 player with about 75 percent of the market. For higher-capacity-memory products such as mobile media players, however, hard drives become a better fit. In fact, small-form-factor hard drives have enabled the high-density mobile media player. Media players offering 4 to 20 Gbytes of storage are the battleground between NAND and hard drives. NAND flash players are now at 4 Gbytes, and this number will grow as more-advanced flash devices become available. NAND flash will gradually capture the market for higher-density players, and whether NAND flash will dominate this market will depend to some degree on consumer tendencies. If they prefer players with moderate storage, say 16 Gbytes, flash will continue to dominate.
While density requirements will go up as media players begin to incorporate video, how much they will rise depends on how the video is used. It is premature to predict which technology will be favored by portable video players.
NAND devices and hard drives have each demonstrated the ability to drastically increase storage capacity, enabling new technologies and applications. Generally, hard-drive applications have larger form factors, big screens and no concern about power consumption. On the other hand, NAND devices are found in appli- cations with small form factors, small screens, robust design and where power is a concern. Naturally, some of these applications overlap, and this is where the two storage technologies are competing for market share. But these markets are not enough to determine the future of either technology. Both NAND and hard drives will continue to ship in volume for the foreseeable future, and though it appears that NAND will gradually take share away from hard drives, new applications and technologies could open new doors for both technologies. *
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