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.