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.

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.

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.

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.

For this blog entry, we’re returning to the Flash Zone, a concept described by Denali Software’s CTO Mark Gogolewski in his keynote speech—The World is Flash: A Disruption of the Memory & Storage Hierarchy—at Memcon 2009. The Flash Zone is the name put to the performance gap between DRAM and disk storage. There’s not only a gap in performance within the Flash Zone, there’s a transition from volatile memory (DRAM) to non-volatile storage (hard disk). With steep cost/bit price declines and per-device capacity growth, NAND Flash devices now easily fit into this gap and produce a new and viable layer in the overall computer memory hierarchy.

What’s new is that Jim Handy’s keynote at the Bell Micro SSD seminar put some welcome numbers on the Flash Zone that further clarify Flash’s place in the hierarchy. Here’s an image of that particular slide.

This image plots the performance and cost of the different memory hierarchy layers from first-, second-, and third-level processor cache through DRAM, disk, and tape. Because Handy’s used a log-log scale to plot everything, the graph looks nice and linear even though the reality is quite a bit messier. For a conceptual graph however, this’ll do nicely.

Note that there’s a gap in the hierarchy. That’s the Flash Zone. Here’s the same plot augmented a bit. The big red circle identifies the Flash Zone.

Also note that Handy has labeled the gap and says it’s “growing.” The gap’s growing because DRAM is getting faster, bigger, and cheaper, moving its ellipse up and to the left while HDDs are getting bigger, although not much faster, moving the HDD ellipse horizontally to the left. The result is a growing performance and bandwidth gap between DRAM and HDDs.

Flash fits into this gap very, very nicely said Handy (and as discussed in this blog previously). Later in his keynote, he displayed this image to underscore the point.

There are currently at least three ways to fill the Flash Zone in a memory hierarchy using NAND Flash memory. The first way, the way that gets the most attention these days, is with solid-state drives (SSDs). Because they employ the same interfaces and share the same form factor with HDDs, SSDs are an easy, drop-in Flash Zone filler. They boost performance just by dropping them into place as HDD replacements, although that may not be the best way to introduce SSDs into the hierarchy. (More about that in a later blog.)

The second way to drop NAND Flash memory into the Flash Zone is through direct- or I/O-attached drives. This is the approach advocated by Fusion-io, as discussed in that earlier AgigA Tech blog entry on the Flash Zone. Direct-attached SSDs eliminate the HDD interface and protocols, which were designed with built-in assumptions about the performance characteristics and limitations of HDDs (“rotating rust” quipped Scott Stetzer, VP of Marketing at SSD vendor STEC). Free of those limiting assumptions and limits, direct-attached SSDs deliver more performance than do SSDs employing HDD interfaces.

Handy showed the ways to introduce these two types of SSDs with the following slide:

In enterprise-class server systems, SSDs with HDD interfaces typically plug into SAN racks and tie to servers over a network while direct-attached SSDs plug directly into the server over a high-speed interface (typically PCIe). Note that smaller servers with HDD interfaces often talk to SSDs directly.

Because he was speaking at an SSD seminar, Handy did not discuss the third way of introducing NAND Flash into the Flash Zone—the approach employed by AgigA Tech’s AGIGARAM. That approach mates the NAND Flash directly to the server’s DRAM, creating a high-bandwidth connection between the two memory hierarchies. In this application, however, the NAND Flash is used for DRAM backup and power-failure bulletproofing—not necessarily for storage (although there are other possibilities to be discussed in this respect).

So far, we’ve only been able to discuss two of Handy’s 47 keynote slides. The talk contained a ton of good information for server designers and enterprise system architects. More later.

Note: Handy’s keynote was based on his company’s new report: Solid State Drives in the Enterprise – 2010.

PCM manufacture involves introducing “foreign” elements (not silicon) from the periodic table into the IC-manufacturing process. Normally, this is something IC manufacturers avoid at all costs, but the material being introduced is glass—albeit something called chalcogenide (pronounces “kal-KAW-gen-ide”) glass—composed of germanium, antimony, and tellurium. The glass is pretty inert, so it apparently doesn’t present too many contamination problems that would absolutely preclude the material’s use in IC manufacturing.

If you’re an electrical engineer, it’s likely you’ve never heard of chalcogenide glass, but it’s one of the most closely studied materials with one of the highest manufacturing volumes in high tech—just not in electronics. A chalcogenide glass layer is the active component in recordable CDs and DVDs. In its crystalline form, the glass is highly reflective. In its amorphous form, the glass is not so reflective, resulting in a nice, binary, optical-storage mechanism. In an optical disk burner, laser-induced thermal heating switches the glass from one state to the other. A fast, strong laser pulse disrupts a spot of sputtered crystalline material and causes it to become amorphous, reducing its reflectivity. You can see the difference if you look closely at a written disk.

These optical differences between the crystalline and amorphous states are essential to recordable, optical-disk operation but they’re not at all relevant to PCM data storage. However, the chalcogenide glass also has measurably different resistance between the crystalline and amorphous states. The crystalline form of the glass has relatively low resistivity and the amorphous form has higher resistivity, until the glass melts. Now you’re talking memory.

You can see the differential resistivity between the crystalline and amorphous states at low “read” voltages in the figure below. At higher voltages, the glass heats and starts to melt. At that point, the crystalline and amorphous V/I curves merge as the glass starts to soften and melt.

PCM cells exploit this V/I curve, which is conceptually similar to the hysteresis curve for magnetic memories. A fast, high-voltage pulse melts a spot of glass in the PCM cell through Joule heating. Joule heating causes a small amount of chalcogenide material to switch from amorphous to crystalline or back again depending on the size and shape of the write pulse (as shown in the following diagram). Once the pulse is removed, quick cooling allows the glass to solidify in amorphous form. A longer, less intense voltage pulse anneals the glass and puts in the crystalline state.

A PCM cell is pretty simple as shown below. The memory cell resides between a bit electrode and a word electrode. The cell itself consists of a current-limiting/heating resistor and a dot of polycrystalline chalcogenide glass. Current flow through this structure and the amount of that current depends on the voltage impressed on the word and bit lines. When the current is high enough, a region of glass next to the resistor (the dark gray mushroom cap atop the resistor in the figure) starts to melt. The PCM chip’s read/write control circuitry controls the size, shape, and timing of the write pulse.

It’s the simplicity of this mechanism and of the PCM cell design that excites chip makers like Intel, Numonyx, and the other vendors chasing after the 4-decade dream of a new form of semiconductor memory.

However, don’t get the impression that this is a trouble-free memory poised to wipe out all existing semiconductor memories overnight. Won’t happen. If this was simple stuff, PCM would have won the semiconductor memory wars long ago. That obviously didn’t happen. Why? Tune in for the next installment.

SSDs are one of three ways to fill the memory/storage gap called the “Flash zone” as discussed in the previous AgigA Tech blog entry, which described Flynn’s initial panel presentation. Although not a major consumer of NAND Flash memory devices, yet, SSD use is growing quickly because of the speed advantages they deliver over what can be achieved with rotating mechanical storage (hard disk drives). Flynn’s talk described the ideal conditions under which I/O-attached storage (including products offered by Fusion-io) can deliver stellar storage performance as measured in IOPS. Flynn’s presentation prompted the first panel question from moderator Coughlin: “Are hard disk drives dead?”

Flynn answered first. Unsurprisingly, he said “No.” Tape hasn’t died either, said Flynn, and neither has DRAM. None of these technologies is in danger of disappearing overnight. HDDs (hard disk drives) currently enjoy a huge cost/capacity lead over any competing storage technology (excluding tape) and HDDs will only disappear when they lose that lead.

Speiser also weighed in. Tape’s huge cost/capacity lead over HDD storage is the only factor that keeps tapes alive for their ultimate use: “offline storage inside of (hollowed-out) mountains.” Tapes will outlast HDDs added Speiser. “They’re the cockroaches of the storage industry.” Chenery, who left HDD vendor Fujitsu in 2006 to start SSD supplier Pliant, also spoke favorably about HDDs. “No one wants a mechanical drive in their computer,” said Chenery, because of the power consumption and susceptibility to physical shock. However, “they provide so much value for capacity” he explained. “In 30 years, who knows?”

Watkins disagreed. “No one cares what’s in their PCs. Consumers think about the applications they want to run. Then they find the best hardware to fit their needs.” Watkins is more concerned by the applications that consumers will be using in five years. His conclusion: all mobility products will evolve into Flash-only use because Flash memory provides superior form factors for small, mobile end products. Meanwhile, cloud storage may obsolete large HDDs in laptops because it’s too dangerous to carry around all that valuable data in a form where it can be lost, stolen, damaged, or destroyed. Yahoo’s Pullara smiled at Watkins’ comment about cloud storage and quipped “How about unlimited storage (in Yahoo’s cloud)? Can you beat that, Google?”

“So if HDDs aren’t going away any time soon,” asked Coughlin, “why did you (Chenery) start Pliant?”

“Because no one would listen to me” replied Chenery, who feels that SSDs are clearly going to redefine they way computer systems are architected.

Speiser jumped on the bandwagon. “We’re looking to invest in companies that have fundamentally rethought applications to back out assumptions based on spinning media.”

Pullara concurred. “Look at anti-spam in 2003” he said. The need to maintain extensive lists of spam sources has soared since then. Maintaining those lists on slow HDDs would make it impossible to reject spam in real time, given the rising volume of spam emails.

Watkins returned the discussion to mobile applications. “The sweet spot for Flash is in the hand,” he said. SSDs must reach 100-Gbyte capacities for netbooks while enterprise applications require terabytes of data storage and corresponding changes in server architecture.

The question of data reliability and trust then arose. Flash memory has well-documented, well-understood wearout and failure mechanisms. In fact, Flash vendors have been far more open and informative about these technology issues than have HDD vendors. As a result, people better understand Flash failure modes and are more aware of them. Chenery grinned and asked “Why would you trust your data to a flying head on a disk?” referring to the incredibly small gap between the read/write head and the spinning media. Head crashes are a well-known HDD failure mechanism. “Flash memory has its idiosyncrasies, but technology overcomes a lot of these” said Chenery. “You manage these idiosyncrasies with appropriate controllers, software, and use models.” In the end said Chenery, system-level designers shouldn’t trust any of the HDD or SSD vendors. They should test and verify reliability claims.

In addition, said Chenery, SSDs don’t “fall off the cliff” (fail catastrophically like HDDs). They provide deterministic, predictable performance that allows for soft failures, usually seen as a gradual capacity decrease as control firmware walls off bad blocks in the Flash memory and moves data to good blocks. Most SSDs will decline in performance over time, claimed Chenery. They must be designed specifically to not decline in performance at the subsystem level. “Getting Flash to deliver deterministic performance in a random environment is hard. It requires enormous computing power.”