By Steve Leibson for Denali Software

The appearance of SSDs into the storage arena is rapidly altering the way large-scale, enterprise-class storage systems are built. Gartner Principal Analyst Sergis Mushell discussed some of these changes at the recent Storage Visions 2010 conference held in LasVegas. Mushell focused on how the introduction of SSDs into the enterprise-class storage device market was reshaping foundation concepts in terms of form factors and interfaces. Mushell started by discussing form factors and projected the graph in Figure 1, which forecasts unit sales of enterprise-class SSDs by form factor:

Figure 1: SSD distribution by form factor (Gartner)

The first thing to note about this graph is the unit-growth forecast for enterprise-class SSDs shown at the top of Figure 1, from 281,000 units in 2008 to 5.3M units in 2013. That’s a 20x increase over a 5-year period and that’s healthy growth is attracting new and existing storage vendors into the enterprise-class SSD market.

Next, notice the relatively rapid decline in the use of 3.5-inch, enterprise-class SSDs. Although these drives constituted the bulk of the enterprise SSD market in 2008, Mushell's chart predicts that 3.5-inch SSDs will become a mere sliver of the market in 2013. At the same time, shipments of 2.5-inch, enterprise-class SSDs appear to stabilize at approximately half of the market. The big percentage growth forecast is for board-mounted SSDs either as PCIe expansion cards or as plug-in DIMMs—not really drives at all.

These forecasts are consistent with three industry trends. First, the unrelenting application of Moore’s Law doubles NAND Flash capacity about every 18 months, so volumetric requirements for the same storage capacity shrink accordingly. Hence the decline of the relatively large 3.5-inch form factor. (Old-timers in the storage industry might crack a smile at the idea that 3.5-inch drives are “large.”)

Second, there’s no particularly good reason why SSDs should be packaged in form factors that are based on the requirements of mechanical rotating storage (hard disk drives, HDDs). Existing HDD form factors are artifacts of platter-size choices and the actual dimensions represent a logical physical progression from 8-inch and 5.25-inch HDDs down to 3.5- and 2.5-inch drives over several decades. Server designers often find these existing HDD form factors to be troublesome. SSDs can assume any convenient or odd form factor because they’re entirely electronic and made of ICs. They initially adopted HDD form factors to allow easy, drop-in replacement of existing HDDs but the use of HDD interfaces is really just another anachronism, as the rapid forecast growth of PCIe-based and DIMM-based SSDs suggests.

Which leads to the third trend and the next graph Mushell displayed, shown in Figure 2:

Figure 2: SSD distribution by interface (Gartner)

This graph forecasts market share for the enterprise-class SSD market by interface type. Note the dominance of Fibre Channel SSDs in 2008, which is consistent with a disk-replacement strategy. Fibre Channel has dominated enterprise-class storage thanks to its high transfer rates but that dominance is ending for both HDDs and SSDs as faster SAS and SATA interface standards appear. Figure 2 forecasts a rapid decline in shipments for SSDs with Fibre Channel interfaces, which predicts that the percentage of enterprise-class SSDs shipped with Fibre Channel interfaces will plummet from nearly 70% of all enterprise-class SSD shipments in 2008 to virtually nothing in 2013. According to this forecast, Fibre Channel’s reign washes away in three waves. The SATA (serial ATA) interface represents the first big challenger to Fibre Channel’s dominance, followed by SAS (serial attached SCSI), and finally by PCIe, which isn’t an HDD interface at all.

SAS and SATA HDD interfaces have been pressed into duty as SSD interfaces for at least two logical reasons. First, these interfaces have become dominant because of their widespread use for all HDD classes, not just the enterprise class, which means that the storage industry has developed a tremendous infrastructure to support these two HDD interfaces. That infrastructure includes everything from driver, OS, and database software; to drive testers; to cables, connectors, and built-in chipset support. In any market, broad infrastructure support and the correspondingly reduced implementation and support costs constitute powerful compatibility incentives that drive vendors cannot ignore.

Second, as HDD manufacturers enter the SSD fray either by developing their own SSD architectures and designs or by buying SSD vendors outright, the use of HDD interfaces on SSDs presents the appearance of a unified product line to customers. Left alone to their own world of storage, existing drive vendors with established HDD product lines have no strong need or wish to differentiate SSDs based on interface type and prefer to offer either type of storage to their customers as plug-compatible alternatives. Established drive vendors’ predisposition to support and maintain legacy HDD interfaces opens the door wide for new SSD vendors that do not have legacy HDD interfaces to support.

As discussed in a previous blog entry, it’s possible to develop SSD architectures that easily outperform HDD interfaces simply by increasing the number of parallel NAND Flash channels in use. It’s not possible to perform the same trick with HDDs. To get higher data rates from HDDs, manufacturers can:

• Spin the disks faster—but at 15,000 RPM, enterprise-class HDD platters are already under severe mechanical stress.
• Increase the number of read/write heads that can be active simultaneously—which constitutes a radical, substantial, and costly architectural and electronic change to HDD design.
• Add a second servo actuator with another set of read/write heads and another set of read/write electronics—which is completely out of the question from an economic perspective.

Consequently, some SSD vendors are beginning to use faster interface standards that are not constrained by disk-centric assumptions that have been baked into standard HDD interfaces including even the latest versions of SAS and SATA. PCIe is a good example of such an unconstrained interface. A 16-lane, Generation 2 PCIe interface provides 8 Gbytes/sec of throughput (much more than SAS or SATA) and a 16-lane, Generation 3 PCIe interface provides 16 Gbytes/sec of throughput. These substantial throughput rates underlie Gartner’s forecast for the eventual decline of all HDD form factors and HDD interfaces in SSD applications.

Related Information:

• Why the Solid State Drive Market is Poised for Growth
• Mr. NAND's Wild Ride: Warning — Surprises Ahead!
• Does MLC flash belong in enterprise SSDs?

Young Choi, Guest Blog for Denali Software

January is a time where lots of planning and forecast are made, with high hopes usually. Semiconductor memory industry, after several years of prolonged downturn, finally started to see some glimpses of recovery lately. Prices are improving, product migrations happening, new process node migration providing production efficiency and hopefully more profitability to the manufacturers.

The information and data that UBM TechInsights has been collecting on commodity DRAM and NAND Flash products clearly show how semiconductor memory industry has been improving their efficiency measured in terms of Mbit per mm2.

For commodity DRAMs, the latest 40 nm class DRAM products show their efficiency of 34Mbit per mm2. When compared to the previous 50 nm class DRAM, 40 nm class shows over 40% improvement. When cost per bit matters the most, 40 nm class DRAM will clearly provide the much needed cost advantage to those manufacturers who have this technology. For those who don’t, they need to find a better way of securing their profitability. When innovation and investment are the name of the game, the gap between the haves and have-nots are obvious, and hence, there is constant movement of joint ventures and merger and acquisitions to create economies of scale. Recent movement of Micron and Elpida, with their respective partner companies in Taiwan, is a clear sign of this. Perhaps later in 2010, we might be able to see the first 4F2 cell based commodity DRAM products. While some DRAM manufactures still have products with 8F2 cell, new 4F2 cell designs combined with smaller geometry would deepen the gap between DRAM makers.

For NAND industry, the trends have been staggering. Introduction of 30 nm class products certainly has contributed to the higher efficiency of commodity NAND Flash products for sure. For NAND, luckily three-bit per cell (X3, TLC or 3-bit MLC) and four-bit per cell (X4, or 4-bit MLC) also helped push the envelope further beyond the lithographical limit in terms of bit density (Mbit per mm2). While all of the major NAND manufacturers (Samsung, Toshiba/SanDisk, Intel/Micron and Hynix) have announced their three-bit per cell and/or four-bit per cell NAND products, there are still some concerns about their reliability and performance. This is reminiscent of the times when MLC (two bit per cell) based NAND products were first introduced. The industry and the market had managed the reliability and performance issues successfully and MLC had become the mainstream NAND technology in many data applications. One can expect that the same would happen to three- and four-bit per cell NAND technology, eventually.

While the past history or performance of the two key commodity memory technologies, DRAM and NAND, has been impressive and even remarkable, the future has a lot of uncertainties. To help us understand what to expect in the future, the presentation which was given by Kinam Kim at Samsung can be a good reference. This was also published on the Semiconductor International website. A patterning limit chart is shown below:

Of course, patterning limit is not the only obstacle to achieve more efficient DRAM and NAND products. There are many other technical challenges for DRAM cell, storage capacitance, isolation, leakage, reliability, floating gate vs. charge trapped flash, double patterning, immersion lithography, so on and so forth. What about new technologies to make DRAM storage cell a thing of past? What about 3D memory?

It appears as though the semiconductor memory industry is following the curves shown above (fairly closely so far). The 40 nm class DRAM products and 30 nm class NAND Flash products that were announced in 2009 are the proof. The real test within the industry will come in 2010. Will we see 30 nm class DRAM in 2010? How about 20 nm class NAND Flash? It remains to be seen but some early signs seem pretty promising. It’s January, a month of high hopes and expectation and a lot of planning for another year. Let’s hope for the best of the semiconductor memory industry in 2010.

PS: 2010 is a New Year for us as a company, too. Semiconductor Insights, which has been a leader in providing technical intelligence and intellectual property professional services to the semiconductor industry is now called “UBM TechInsights”.

For various DRAM and NAND Flash analysis reports (process analysis, circuit analysis and waveform analysis/functional testing) on the latest 40 nm class 1Gbit DDR3 SDRAM, 30 nm class 32Gbit MLC NAND Flash, 30 nm class 32Gbit Three-bit per cell NAND Flash, 40 nm class 32Gbit Three bit per cell NAND Flash, please visit UBM TechInsights’s Open Market Reports page.

By Steve Leibson for Denali Software

IDC’s Research Director John Rydning and Micron’s Director of SSD Marketing Justin Sykes tackled the merging abilities of fast enterprise-class SSDs and evolving disk interface standards, particularly SATA 6G (also called SATA 6.0) and USB 3.0, while speaking on a panel about the technology of storage during the Storage Visions 2010 conference held early this year in Las Vegas. Rydning spoke first and he compared and contrasted two new external disk-interface standards, namely USB 3.0 and eSATA 6.0. These standard disk interfaces improve on their predecessors. USB 3.0 maximum data rates are 3.2 to 4.8 Gbps versus USB 2.0’s 480 Mbps—a 6.7x to 10x boost in theoretical I/O performance. SATA 6.0 and eSATA 6.0 essentially double the theoretical maximum data rate of SATA 3.0 and eSATA 3.0 from 3 Gbps to 6 GBps. Consequently the new SATA 6.0 and eSATA 6.0 interfaces are theoretically faster than the new USB 3.0 interface just as SATA 3.0 and eSATA 3.0 are faster than USB 2.0.

The SATA and USB standards seem to be in lock step with respect to adoption rates according to Rydning. He showed comparison graphs that forecast increasing adoption rates for both SATA/eSATA 6.0 and USB 3.0, with some minor amount of adoption in 2010 and about 50% market penetration for each interface by the year 2012.

To aid this transition, laptop makers have started to build eSATA interface ports into laptops. This is not a particularly difficult feat because most motherboard chipsets include several SATA ports so implementing an eSATA port for such a machine is a matter of adding an eSATA connector to the laptop motherboard. For desktop and enterprise-class server systems, adding an eSATA port requires little more than a SATA extension cable that connects the motherboard SATA connector to an eSATA connector mounted on a metal expansion-card bracket or a case bulkhead because SATA ports are plentiful on most desktop and server motherboards. Rydning also pointed out that officially, eSATA connectors supply no power to the external SATA drive but connector manufacturers have developed an “unofficial” hybrid eSATA/USB 2.0 connector that allows a properly designed cable to tap into the co-located USB port’s 5V power while simultaneously coupling the eSATA disk-interface signals to the external drive.

Sykes’ panel presentation corroborated Rydning’s and provided some important test data to reinforce some of Rydning’s points and to make new ones. First, Sykes presented a historical chart showing the uneven throughput progress for SCSI and ATA disk interfaces as they evolved into the SAS (serial attached SCSI) and SATA (serial ATA) interfaces.

SCSI/ATA/SAS/SATA disk interface data rates over time (Micron Technology)

The graph shows that the SCSI disk interface led in throughput until both SAS and SATA interface standards hit 3 Gbps around 2005. With the development of a 6 Gbps standard in 2008, the SAS interface pulled ahead of the SATA interface and will remain in the lead even with the development of the new SATA 6.0 specification.

Sykes then showed a different sort of performance graph for an existing MLC (multi-level cell) SSD using SATA 3.0 and SATA 6.0 interfaces:

MLC SSD performance with SATA 3.0 and SATA 6.0 interfaces

The graph shows that sequential reads for this particular SSD benefit greatly from the faster interface although the read speed does not double with a doubling of the interface transfer rate. This result indicates that the SATA 3.0 interface definitely limits this SSD’s read performance. Although the SSD’s random read performance benefits some from the faster disk interface, the SSD’s sequential and random write performance essentially gains nothing from SATA 6.0.

These figures could lead you to the wrong conclusion, so take care in your interpretation. What the above figures do show is that the drive being tested was designed and optimized for the SATA 3.0 interface. In other words, the number of NAND Flash channels implemented in the tested drive is sufficient to support the SATA 3.0 data rate. Slapping a faster interface on this existing SSD architecture doesn’t produce a substantally faster SSD. To fully exploit the faster performance abilities of the SATA 6.0 interface, SSDs need more internal NAND Flash channels to boost internal read/write parallelism. That’s what Sykes’ next graph depicted:

Boosting NAND Flash channels increases SSD performance to SATA 6.0 rates
(Micron Technology)

Increasing the number of NAND Flash channels implemented in an SSD substantially increases the SSD’s read and write speeds (using either multi-level-cell or single-level-cell NAND Flash devices). In fact, the theoretical performance of SSDs that support 16 or 32 active NAND Flash channels greatly exceeds the bandwidth of 6-Gbps disk-interface standards, which means that the SAS and SATA disk-interface standards will need to evolve even further to keep pace with future SSD developments.