2x nm will happen – Currently, vendors are shipping 34nm NAND Flash memory in volume. That’s a tremendous engineering feat in itself. As reported in an earlier blog entry (“Intel, Micron striving to regain lead in NAND tech”), Intel and Micron Technology are apparently planning to ship NAND Flash devices based on 2x nm lithography (called 2x because it’s not yet clear if it’s 26nm, 25nm, 24nm, or 22nm). Lithography shrinks are a true race to the bottom. Features on chips have become so small that one or two atoms of difference from one part of a chip to another cause real changes in device characteristics. This phenomenon is called on-chip variation or on-chip variability (OCV) and it’s a tough problem to tackle, requiring the use of smarter on-chip circuitry to deal with the variation. (See “My Head Hurts, My Timing Stinks, and I Don’t Love On-Chip Variation” by Matt Weber.) The problems do not appear to be insurmountable and NAND Flash vendors currently leading the lithography derby appear intent on keeping that lead until it’s no longer possible.

SDDs will get bigger and faster – This prediction needs to be written, but it’s really a no-brainer. The “bigger” part of the prediction is based on the ever-increasing capacity of the NAND Flash chips used to make SSDs. They will get bigger, driven by economic forces far beyond SSD usage. USB Flash-memory drives are the big volume driver in this market and there’s always demand for more capacity there while the form factor emphasizes small and slim. Nowhere to go but more on-chip capacity. Number two driver is SD cards for cameras and camcorders, with the same forces at work. As for faster, it’s clear that SATA 6G is in the immediate future for SSDs. Marvel’s SATA 6G controller (see “Early Results Show SATA 6G Performance All Over the Map”) and Micron’s introduction this month of an SSD that exploits SATA 6G to good effect (see “New SSD Introductions from Seagate and Micron”) clearly point the way to faster SSD operation, once the internal architectural designs are aligned with the faster interface.

ONFi 2.1 gets big – ONFi stands for the “Open NAND Flash interface” and the ONFi organization (www.onfi.org) bills it as the fastest Flash interface on the planet. Hyperbole aside, ONFi supports transfer rates to 200 Mbytes/sec. and that’s speedy in anyone’s book. The ONFi Working Group was formed in May 2006 and currently has over 80 member companies including Hynix Semiconductor, Intel Corporation, Micron Technology, Inc., Numonyx, Phison Electronics Corporation, Sony Corporation and Spansion. Wanna fight those guys as a group?

MLC and TLC get big – It’s already happening. Several NAND Flash vendors already offer MLC (multi-level cell) and TLC (three-level cell) NAND Flash devices. They are working to improve the reliability of these devices while SSD and other subsystem manufacturers are working to develop system-level techniques to mask the reliability of these devices. That’s not a patch job. HDD vendors have spent the last 50 years paving over the data-reliability problems of rotating magnetic storage and no one pays much attention any more except the engineers tasked with keeping those problems at bay. Much the same will happen for NAND Flash devices and for products based on those devices.

NAND Flash prices firm up – NAND Flash prices have recently risen and a lack of capital investment in new fabs and processing equipment foretells the usual period of spot shortages and price peaks associated with scarcity. See MemoTrek’s extensive analysis: NAND Flash Prices: 4Q Trends & 2010 Forecast.

Joker’s wild – If we told you, then it wouldn’t be a surprise, would it?

Please have a happy and safe New Year’s celebration and we’ll see you next year.

Seagate announced its Pulsar SSD line on December 7 or 8 (depending on which version of the press release Google finds for you), allowing a show to drop that people had expected for more than a year. Pulsar drives use the familiar 2.5-inch HDD form factor and a SATA interface, making it easy to drop the drives into existing computer and server systems. Seagate’s Pulsar SSDs employ SLC (single-level cell) NAND Flash devices, which cost more per bit than MLC (multi-level cell) and TLC (three-level cell) NAND Flash devices. In exchange for the higher cost, you get more reliable memory, as was discussed in this blog a while back. (Check out “More than Moore: SLC, MLC, and TLC NAND Flash.”)

Seagate Pulsar SSD

The use of SLC NAND Flash underscores Seagate’s focus on enterprise-class storage for the SSD. There are at least two good reasons for Seagate’s enterprise focus. First, enterprise customers are more able to translate an SSD’s speed advantage over HDDs into dollars (as previously discussed in the blog entry “SSD TCO (Total Cost of Ownership”). Second, SSDs are a premium product with a premium price. Enterprise customers more easily accept the higher cost/Gbyte price tag attached to SSDs. Seagate’s Pulsar SSDs are available in storage capacities to 200 Gbytes and the SSDs achieve “a peak performance of up to 30,000 read IOPS and 25,000 write IOPS, 240MB/s sequential read and 200 MB/s sequential write” according to Seagate’s press release. The Pulsar drives have a 5-year limited warranty.

Micron Technology rolled out its RealSSD C300 less than a week before Seagate’s SSD announcement. The first glaringly obvious difference in Micron’s C300 SSD is that it sports a 6-Gbyte/sec SATA 6.0 interface. However, the faster interface alone will not boost performance (discussed earlier in this blog here) if the drive internals aren’t designed to sustain high transfer rates supported by SATA 6.0. To that end, Micron’s RealSSD C300 press release discloses the fact that the new Micron SSD “leverages a finely tuned architecture and high-speed ONFI 2.1 NAND Flash  to provide a whole new level of performance.” (ONFi, the Open NAND Flash interface, is discussed in this previous blog entry.) The result: a read throughput speed of up to 355MB/s and a write throughput speed of up to 215MB/s.

Compare those numbers to Seagate’s Pulsar and you’ll see that the Micron drive’s read throughput is nearly 50% faster but the write throughput is only 7.5% faster. Write throughput is one of the Achilles’ heels of SSDs. NAND Flash devices had an erase/write cycle that simply takes time.

Micron RealSSD C300

Micron’s C300 SSDs will be offered in 1.8-inch and 2.5-inch form factors, with both form factors supporting 128- and 256-Gbyte capacities. Micron is currently sampling the C300 SSD in limited quantities and expects to enter production in the first quarter of calendar 2010.

Both companies are making smart moves into the SSD market. Seagate, like Western Digital and its acquisition of SSD vendor SiliconSystems in March of this year, recognizes that it’s not in the HDD business—it’s in the storage business and SSD storage is hot right now. Micron, like Intel, sees SSDs as a value-added way to package and market it’s NAND Flash devices. Both companies have made very smart moves into the SSD market.

• More density
• More speed
• Less cost
• Less power

Depending on the specific application, one or two of these major factors may be more important than the other, but they’re all important factors—all of the time.

Currently, we have divided the use space for semiconductor memory into four big regions:

• SRAM serves applications that require frequent, fast data access—more speed (usually cache)
• DRAM serves applications that need large storage space—more density—for frequently changing data at a low price—less cost
• NOR Flash currently serves the spot for holding code and data that must be accessed quickly—more speed—but doesn’t change often. You see NOR Flash mostly in smaller embedded applications because larger embedded applications, computers, and servers combine hard disk drives (HDDs) or solid-state disks (SSDs) with DRAM to serve the same function instead of NOR Flash.
• NAND Flash is a story all by itself. As an industry, we use NAND Flash in a wide variety of ways. We use it to hold data and code in bulk because it’s the cheapest semiconductor memory available—less cost. At the same time, NAND Flash is non-volatile, so it’s useful for retaining information through power outages. That’s why SSDs are packed with NAND Flash chips and it’s also why AgigA Tech uses NAND Flash to back up DRAM in its AGIGARAM bulletproof Non Volatile System (NVS) memory modules. Even better, NAND Flash power consumption is fairly low—less power—if the system uses the NAND Flash memories infrequently, which is exactly how they’re applied in AGIGARAM NVS memory modules.

Memory Technology Inflection Points

The immense importance of memory in a processor-centric, multicore world results in tremendous technology R&D efforts to develop semiconductor memories that improve on one or more of the four major factors listed above. Memory storage technology and memory cell design get a lot of attention. One aspect of memory design that sporadically pops up in importance is the memory interface.

For some, the memory interface isn’t nearly as glamorous as a new kind of memory cell (think phase-change memory or PCM, which has held the limelight lately) or lithographic shrinks (think 32nm heading for 2x nm). However, the memory’s interface performance plays a major role in determining how a memory performs and even how much power it consumes.

In the world of NAND Flash, ONFi (the Open NAND Flash interface) and the Toggle-Mode NAND interface are coming to the fore. We’ll leave the discussion of these competing, high-speed NAND Flash interfaces for another blog post. Today’s topic is DRAMs. For DRAMs, the hot “new” interface is DDR3, which is the third major iteration of the JEDEC interface standard for synchronous, double-date-rate (DDR) DRAM.

The original DDR (double data rate) specification appeared in June, 2000 after a four-year gestation. The DDR memory interface replaced the original JEDEC SDRAM interface, which appeared in 1993. Before that, DRAM used the baroque RAS/CAS asynchronous control structure and multiplexed row/column address lines that Mostek developed for the MK4096 4-kbit DRAM in 1973 to reduce package pin count. That old RAS/CAS stuff is still there, deep inside of today’s advanced DRAMs, but it’s now buried inside of the DDR parts where you can’t see it unless you’re a DRAM chip designer.

DDR3 Memory’s Advantages

What are the advantages of DDR3 over DDR2? They go straight back to the four major factors listed at the beginning of this blog post. Compared to DDR2, DDR3 memory provides more speed, more density, operates at lower voltage (and therefore consumes less power), and it will be less expensive than DDR2 memory at some point in the coming year. In short, DDR3 memory improves on all four of the major factors relative to DDR2 memory. Bets don’t get much safer than that.

For enterprise-class systems (servers), DDR3 memory provides many specific advantages. First, it promises denser memory modules by accommodating DRAM chips as large as 16 Gbits, permitting the development of 16-Gbyte registered DIMMs. Enterprise-class server architects love denser memory modules because they’re always strapped for room inside of their server boxes. DIMMs take up space and, worse, they block air flow and make cooling more difficult inside of the enclosures. Fewer DIMMs is definitely better for air flow.

Second, DDR3 memory transfers twice as much data per clock as DDR2 memory. Enterprise-class server architects can use this speed in one of two ways. They can run their processors faster with faster memory or they can run at the same speed but cut the clock rate to the memory modules and thus cut power consumption.

Real Power Savings

But the real power savings comes from DDR3’s lower operating voltage. DDR2 memory is specified for a 1.8V operating voltage while DDR3 memory operates at 1.35V (and maybe 1.2V in future low-power DDR3L devices). Because operating power is proportional to the square of the operating voltage, that small 150mV drop between DDR2 and DDR3 operating voltages translates into an appreciable drop in operating power—about 30% less!

Enterprise-class server designers like lower operating power, therefore less waste heat. In fact, they like it a lot. That’s because data centers pay double for every excess Watt of server power. Roughly speaking, each Watt consumed by a server takes one Watt of electricity to run and another Watt to cool the server. By at least one estimate, DRAM power usage accounts for 25% to 40% of a data center’s energy costs (and can be more than 50% according to this Denali memory blog post). By another estimate, Google’s power costs were $50 million in 2006. So power reduction is very high on the server designers’ wish lists because data-center operators can easily translate reduced power consumption into monetary savings and they are quite aware of that sort of calculation with respect to total cost of ownership (TCO) when evaluating competing servers.

Perhaps the biggest force driving the adoption of DDR3 memory is the support of Intel and AMD. Intel’s Core i7 and AMD’s Phenom II multicore processors and chipsets presume DDR3 memory. It won’t take long before this presumption filters down to the lesser PC processors and PC processors are the big dogs in the memory kennel. They largely drive what happens with mainstream DRAM parts. So DDR3 memory’s success is likely assured, just as DDR2’s was before that and just as DDR memory supplanted SDRAM. The cycle repeats, and often.

Currently, AgigA Tech offers AGIGARAM NVS modules with SDRAM and DDR2 interfaces. It doesn’t yet offer an off-the-shelf DDR3 module, but given the industry’s track, you can safely bet that there’s a DDR3 AGIGARAM module on the road map.

Find those last two sentences confusing or contradictory? Here’s the explanation.

SSDs have no read/write heads or positioning arms. Instead, they consist of several NAND Flash chips and a controller chip. There’s an array of memory blocks on each NAND Flash chip. The size of the Flash memory block is the smallest amount of memory a NAND Flash chip can write in one operation because NAND Flash memory blocks are atomic with respect to erasure. You can’t write just one byte or word because you must erase the entire block before writing to the block. That means an SSD can only write an entire NAND block at a time.

Here’s a graphic from Jim Handy’s SSD keynote at the Bell Micro SSD seminar held in early December that helps to explain the situation:

Each SSD consists of a stack of visible NAND memory blocks that the SSD controller uses to store written data. There’s also a shorter stack of spare NAND memory blocks that hold data in temporary storage. These spare blocks are also used to replace a visible block when it wears out from repeated write/erase cycles. All NAND blocks are equally accessible, so there’s no time penalty for writing NAND blocks out of sequence as there is when writing on non-adjacent or non-contiguous tracks with HDD storage.

However, most virtual operating systems don’t write in blocks, they write in 4-Kbyte pages that are much smaller than NAND Flash blocks. For example, Numonyx’ 1-to-16-Gbit NAND Flash devices have 128-Kbyte blocks. As a result, modifying one 4-Kbyte page in a NAND Flash block requires a relatively complex sequence:

1. Read the data for the entire block from NAND Flash into a RAM buffer
2. Modify the appropriate page in the block image now stored in RAM
3. Write the block back to an erased NAND Flash block
4. Fix pointers to the new memory block
5. Erase the old memory block as a background task

Consequently, SSD performance varies over time and the performance varies depending on how many erased and spare memory blocks are available across all of the NAND Flash chips in the SSD. SSD performance also depends on the ratio of reads versus writes—because reads occur ten times faster than writes for SSDS—and they vary over time as the NAND Flash chips fill up.

The following figure from Handy’s keynote shows a 3D data surface plot representing the IOPS performance of one SSD. (The figure is from a presentation at the August 2009 Flash Memory Summit made by Esther Spanjer, Director of SSD Marketing at Smart Modular Technologies.)

The X axis of the surface shows the ratio of reads to writes and varies from 100% writes on the left to 100% reads on the right. The Y axis shows SSD performance in IOPS. The Z axis plots “block” size, from the SSD-level perspective (which is page size from the NAND Flash chip’s perspective, yes that’s confusing).

The first thing to note from this surface plot is that performance is a lot better on the right-hand side, which is dominated by reads. You’d expect that because SSD read performance is 10x better than SSD write performance. It’s the nature of NAND Flash memory. Note how fast the performance falls off as the percentage of write transactions increases. Then note that there’s a sort of saddle effect along the Z axis. The saddle peaks at 4-Kbyte blocks. Most SSD designs are optimized for 4-Kbyte blocks because most virtual operating systems employ 4-Kbyte blocks (and have for decades, in spite of the radical, orders-of-magnitude increase in memory use by both operating systems and application software).

So, clearly, when an SSD vendor gives an IOPS rating for an SSD, you need to take that one number with a grain of salt. SSD performance varies significantly depending on the read/write mix and on block size. Consequently, SSD performance can’t be captured in one or two numbers.

Next, Handy presented this graphic from SandForce (which makes SSD controller chips):

This graph shows an initial conditioning period during which the test preconditions (fills up) the SSD using sequential 128-Kbyte writes. The initial transfer performance (about 80 Mbytes/sec for the particular drive being tested) drops slightly as the drive fills and the internal SSD controller starts shuffling full NAND Flash blocks off to spare memory. The falloff isn’t big because the sequential writes place a predictable load on the SSD controller. However, when the test switches to random 4-Kbyte writes about 4000 seconds into the test, performance drops significantly because the SSD controller suddenly needs to make small changes to memory stored in the NAND Flash blocks but the drive’s full and there are no empty blocks. Blocks must be erased to make room for the new data and block erasure takes time. Consequently, there’s a big performance falloff as the controller starts to shuffle data around inside of the drive to make room for new data.

Perhaps more interesting is what happens when the test switches back to large sequential writes about 11,000 seconds into the test. Initially, the sequential writes cause the drive performance to vary wildly because the preceding random writes have scattered the spare blocks and left them distributed throughout the SSD’s internal NAND Flash memory space. Eventually, the SSD’s internal controller gets things sorted out and the performance for large sequential writes returns to the initial steady-state level.

(Note: This graph is not supposed to typify the performance of all SSDs. The graph shows the results of a test on one particular SSD.)

So what’s to be learned from all of this data? SSD performance measurement isn’t simple. Creating controllers and firmware that deliver optimum SSD performance isn’t simple either. As drive and chip vendors learn more about the use of NAND Flash for storage, they develop better algorithms for extracting more performance from the NAND Flash chips.

NAND Flash chips are complicated, whether used in SSDs or for server memory backup as with AgigA Tech’s AGIGARAM modules. It takes experience to get the most performance from these memory devices.

My thanks to Jim Handy for all of the great information in his Bell Micro keynote, and for generously letting me use the information in this series of blog entries.

First, take a look at this graphic depicting a “typical” server storage array consisting of 100 enterprise-class HDDs.

Each short-stroked, enterprise-class HDD has a capacity of 300 Gbytes, for a total array capacity of 30 Tbytes. This enterprise-class HDD array delivers 30K IOPS, costs $55,000, and consumes 1.75 kilowatts of electricity (not to mention an equivalent amount of electricity required for cooling). That’s the baseline.

Now look at this graphic, which compares the previously discussed array of enterprise-class HDDs with a hybrid array consisting of one SSD and 30 high-capacity, 1-Tbyte HDDs.

The high-capacity drives are not short-stroked, so they can provide a total storage capacity of 30T bytes with only 30 drives instead of 100 enterprise-class HDDs. However, the one SSD inserted into the drive array provides the same IOPS read performance as the 100 enterprise-class HDDs, so the use of the slower, less expensive, high-capacity HDDs in the second array is not a detriment to the second array’s IOPS performance, as long as the server software is written to make use of the hybrid array’s abilities.

The SSD-enhanced drive array costs $6040 or about 90% less than the array of 100 enterprise-class HDDs. The SSD-enhanced array consumes 0.392 kilowatts, which is nearly 80% less than the enterprise-class array of 100 short-stroked HDDs. Consequently, the second drive array generates substantially less waste heat (that must be cooled) than the full array of enterprise-class HDDs.

As a result, the SSD-enhanced drive array saves the enterprise customer a substantial amount of money when viewed from a systemic perspective. Relative to the enterprise-class HDD array, the SSD-enhanced hybrid drive array costs less to purchase; costs less to provision because fewer drives require less rack space and fewer racks; consume less electricity for operation; need less electricity for cooling because fewer, slower drives generate less heat; and reduce maintenance costs because the high-capacity drives run cooler (increasing MTBF), because there are fewer drives to maintain, and because high-capacity drives are much less expensive than enterprise-class drives. Overall, the TCO calculations favor the SSD-enhanced, hybrid drive array.

Handy took Sun’s numbers a step further by calculating the crossover point where TCO considerations favor an SSD-enhanced drive array over an array of enterprise-class HDDs when the IOPS performance is the main consideration rather than capacity. Here are Handy’s graphs:

The left graph shows price curves for an array of enterprise-class HDDs versus an array of SSDs. The array of SSDs initially costs more than the enterprise-class HDDs, so the crossover point is 1200 IOPS due to the high initial SSD cost. As the IOPS requirement rises, you need to add enterprise-class HDDs to the array to meet the higher IOPS requirements but one SSD gets you a lot of IOPS so there’s no need to add one until the IOPS requirement exceeds around 3000 IOPS. For the enterprise-class hybrid array, which mates one SSD with several high-capacity HDDs, the purchase cost of the SSD-enhanced array is much lower for a given capacity so the crossover point is also lower—just 400 IOPS.

TCO computations such as these are required for storage and for memory subsystems. It’s easy to be myopic and compare component cost to component cost, but system architects are creating systems and should always try to view component costs through a TCO lens.

As you can see from the graph, an enterprise-class SSD has more than twice the read bandwidth of an enterprise-class HDD and more than three times the read bandwidth of a “capacity” HDD. The enterprise-class SSD bests the enterprise-class HDD’s write bandwith by only 10%, reflecting the slow write/erase cycles of NAND Flash compared to the NAND Flash read latency.

The IOPS line in the graph is slightly misleading. While SSDs may perform 35,000 IOPS for reads, they’re about 90% slower for write IOPS. In reality, there’s a mix of reads and writes in any system, so real-life IOPS performance will fall somewhere between the extremes.

However, focus on the last line of the graph for a moment. This line describes latency and it’s here that SSDs truly shine due to their lack of mechanical components. A “capacity” HDD has a 15-msec latency, mostly due to the average amount of time required to move the read/write head from one disk track to the next. The short-stroked enterprise-class HDD has been restricted so that head seeks are radically reduced. The benefit is a nearly 10:1 reduction in latency, from 15 msec to 2 msec. The cost is a huge loss in disk capacity. Yet many enterprise system designers are willing to pay this cost to get the huge latency cut.

But look at that last number—the latency number for an enterprise SSD. There’s no mechanical head to move. There’s no disk platter to rotate into the right position. There’s just a large array of addressable NAND Flash blocks, all accessible in essentially the same amount of time. Latency is one of the biggest SSD benefits and it is one reason why server architects are beginning to intermix SSDs and HDDs to balance capacity with speed.

You can see that from 2007 to 2008, NAND Flash pricing was in freefall and dropped from nearly $9/Gbyte to less than $2/Gbyte. This drop had two big effects. It hammered the NAND Flash vendors and it enticed a lot of companies to jump into the SSD market with visions of even cheaper NAND Flash chips on the horizon.

For now, it looks like NAND Flash pricing is on a more manageable price decline through 2012, at least according to this Forward Insights forecast. While this easing of the rate of price decline doesn’t bode well for those who look forward to large future price drops for SSDs, it provides hope for some bedrock stability for other NAND Flash applications such as AgigA Tech’s server-class, bulletproof AGIGARAM memory modules.

This first image from the article compares the observed average read performance from a Seagate XT SATA 6G HDD, a Seagate Barracuda SATA II HDD, and one of Intel’s X25 solid-state drives (SSD). The benchmark being used here is Simpli Software’s HDTach.

You can see that the SATA 6G drive is about 6 to 7% faster on the benchmark than the 3-Gbits/sec SATA II drive. That’s a far cry from twice as fast, strongly suggesting that the HDD interface is not the limiting factor for HDD performance, at least not in this situation. However, take a look at the performance of the Intel X25 SSD, with a SATA II interface. Its average read bandwidth is about 70% better than the Seagate SATA II HDD and 60% better than the Seagate SATA 6G HDD.

Now the impetus for this PC Perspective article was the receipt of a very unusual SSD from Marvell. Marvell is a semiconductor vendor. Unlike Intel, Marvell doesn’t make SSDs; it makes SSD controller chips and this Marvell SSD, which contains a Marvell SATA 6G SSD controller chip, is an engineering sample designed to help system developers evaluate SATA 6G for their systems.

According to the article, this Marvell SSD isn’t built with NAND Flash devices. It’s built with ROM devices. So you can read from it but cannot write to it. It’s a read-only SSD, which is not particularly practical if you’re building computer systems but this drive makes a good enough tool if you simply need to exercise or evaluate SATA 6G interfaces.

So how does the Marvell SATA 6G SSD fare? Here’s the graph from the PC Perspective article:

The Marvell read-only SATA 6G SSD attains a burst-read rate of just over 350 Mbytes/sec while the Intel X25 SATA II drive attains a burst-read rate of just over 260 Mbytes/sec. So the burst-read rate for the Marvell SSD is about 1/3 faster than for the Intel X25 SSD. Unfortunately, because the Marvell SSD is a read-only device, PC Perspective could not compare burst-write rates, which tend to be significantly slower for SSDs. Consequently, you might expect that the SATA 6G interface won’t be so helpful for write transations.

What to conclude?

Well, first of all, PC Perspective comments that the Intel SSD appears to be close to saturating the SATA II interface, which speaks well of the Intel X25 SSD’s internal architecture. Next the results indicate that SSDs will disproportionately benefit from the faster SATA 6G interface than will HDDs. Finally, it suggests that future SSDs designed for the faster SATA 6G interface standard will need to employ more than the 10 NAND channels employed in the Intel X25 SSD to boost the internal bandwidth of the SSD architecture.

Why are the solid state disk drives still so expensive?

They are on the market for years and still so expensive. SSD of a reasonable capacity (256GB) costs as much as $800 or more. Aren’t they going to drop the prices?

Although the question appears to have been posed by someone not closely familiar with the ins and outs of hard-disk drive (HDD) and solid-state disk (SSD) technologies, markets, and pricing, it’s a frequent question posed by many in the industry. We’ve become so accustomed to large, regular drops in price/capacity for both mechanical storage (“rotating rust”) and semiconductor memory that we’ve collectively developed a sense of entitlement. If we can’t buy it today, we think, surely the price will drop and we’ll be able to afford it soon.

However, when we compare the price/capacity of SSDs against HDDs, we’re comparing one moving target against another. Moore’s Law governs the price of SSDs because the largest cost component in an SSD is NAND Flash memory (see below). Moore’s Law has been a monster force in the semiconductor industry, pushing prices ever lower for more than four decades. However, the HDD vendors are constantly working with their own price-reduction curve, which has proven to be just as robust as Moore’s Law. By pulling a veritable menagerie of rabbits out of various technological hats, HDD vendors have dropped per-bit pricing for HDDs about as fast as semiconductor vendors have cut the price/bit of NAND Flash memory.

Take a look at this graph from Handy’s keynote:

From the gross slopes of the two curves, you can see that HDD cost/capacity has remained about 20x lower than NAND Flash memory cost/capacity throughout this decade. Note that in 2006, there was a serious downturn in the slope of the curve for NAND Flash. Extrapolating that new slope led some to predict that NAND Flash cost/Gbyte would cross over that of HDDs by 2008 or 2009. That just didn’t happen. The increased rate of price decline was economically unsupportable and caused huge turmoil among NAND Flash vendors. (For extensive analysis of this situation, see this blog entry on Denali Software’s Web site.)

Now please understand, the expectation that NAND Flash cost/Gbyte would zoom past the HDD cost/Gbyte curve wasn’t just wishful thinking. NAND Flash per-bit costs did overtake and then zoom past that of DRAM, which was once the semiconductor industry’s king of cost/bit. That event happened in 2004 as shown in this slide from Handy’s keynote.

So the expectation that NAND Flash cost/bit would zoom past HDD cost/bit wasn’t at all far-fetched. It just didn’t happen. HDD vendors happily continued to cut the cost/bit of rotating storage, to the very great benefit of consumers and enterprise users everywhere.

Handy’s simple silicon anatomy of an SSD shows why the SSD’s cost/bit is closely tied to the cost of NAND Flash.

From a silicon perspective, Handy’s illustration shows 34 key semiconductor devices in his example 64-Gbyte SSD. Two of the devices are a controller chip and a DRAM buffer. Total cost for those two devices: $6. The other 32 devices are NAND Flash chips. Total cost for those devices: $64 for 64 Gbytes of storage (not counting spare capacity). The cost of the NAND Flash devices is more than 90% of the silicon cost of an SSD. The SSD’s price is largely set by the cost of its internal NAND Flash.

That’s why SSDs aren’t likely to replace HDDs for bulk storage in the foreseeable future. As long as the HDD industry has a road map leading to higher capacity and lower cost/bit storage, and it does, then the HDD will keep the throne as the storage capacity king.

SSDs can beat HDDs in raw performance by one or two orders of magnitude, as measured in IOPS. There’s nothing on the HDD road map that can change that situation. For applications that can measure the value of storage speed, and there are many such applications for enterprise-class storage, SSDs provide sufficient value to justify their higher price/bit. For most consumers, people who are selecting laptops for example, the choice between a 160-Gbyte HDD or a 32-Gbyte SSD for the same price is obvious. The consumer will choose more capacity (to store more music, more pictures, more video, and more movies) every time.

Now take a look at Handy’s curves for DRAM and NAND Flash cost/bit once again:

Note that the cost/bit of NAND Flash is now roughly 10% that of DRAM. That means that as a DRAM backup medium, NAND Flash doesn’t add that much to the cost of the DRAM it’s backing up. Unlike the comparison of NAND Flash and HDD capacity, which tilts far in favor of the HDD, NAND Flash densities are much better than DRAM bit densities and that gap is growing thanks to multi-level cell (MLC) storage. These economics are behind the idea for AgigA Tech’s AGIGARAM modules. For a small cost adder, volatile DRAM can be made bulletproof when paired with NAND Flash memory. For more detail regarding this idea, see the earlier 3-part series in this blog (here, here, and here).

One of the interesting facets Handy discussed was the current practice of short-stroking enterprise-class hard disk drives (HDDs)—“abusing” them, as Handy explained. The idea’s pretty simple. An HDD’s average access time is determined by the average amount of time it takes to swing the arm carrying the read/write heads into position plus the average rotational latency. The fastest enterprise-class HDDs now spin at 15,000 RPM so there’s not much room for trimming there—not without having the disk platters fly apart under the centripetal force. However, there’s something that can be done about the average seek time. Simply use fewer of the available tracks on the disk. Doing so, you get faster average seek times because the arm never needs to travel very far.

You pay for that decreased seek time with lost capacity. You simply don’t use most of the tracks and therefore you discard most of an HDD’s storage capacity.

Handy gave the following real-world example of such HDD abuse. He described IBM’s DS8300 Turbo. It has best-in-class TPC-C specs: 123K IOPS, 16-msec latency. It gangs 512 HDDs—consisting of 73- and 146-Gbyte enterprise-class drives—into mirrored RAID arrays. The result is a storage subsystem with 53 Tbytes of actual capacity, but short-stroking the drives reduces the usable capacity to 9 Tbytes. IBM threw away 83% of the raw capacity to get those best-in-class TPC-C performance specs.

This is yet another example of why SSD manufacturers are crowding into the Flash Zone. If IBM can afford to throw away 83% of the available capacity in a huge multi-multi-Tbyte bank of enterprise-class HDDs, then high-performance SSDs that can muster one or two orders of magnitude performance improvement relative to the “rotating rust” HDDs must be worth a lot of money to data-center architects.

And apparently, they are.