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Storage tiers: SAS interface, MLC flash rising

SAS vs. SATA battle in enterprise data storage shows hard drives with SAS interface trending up; cheaper MLC SSDs outnumber SLC in enterprise.

The latest trends in enterprise storage drives reflect the tiering strategy that many IT shops are taking to address the explosive growth of unstructured data. Those who use solid-state drives for their highest I/O needs increasingly favor cheaper multi-level cell over single-level cell. And, when they're not spending extra money for SSDs, they use 10,000 and 15,000 rpm SAS hard-disk drives for performance and 7,200 rpm SATA drives when capacity is paramount.

That leaves the Fibre Channel (FC) interface for both hard-disk drives (HDDs) and solid-state drives (SSDs) on the verge of extinction. Once the HDD of choice for an IT shops' highest performance needs, FC drives last year accounted for only 2% of HDD shipments and 2% of SSD shipments, according to IHS iSuppli Corp., a division of IHS Inc. in Englewood, Colo. By 2014, they'll be gone; save for negligible amounts, IHS iSuppli predicted.

John Rydning, a research vice president focusing on HDDs and semiconductors at Framingham, Mass.-based International Data Corp. (IDC), said via email that he expects FC HDDs to hang around for two more years as a replacement for capacity-expansion drives for FC storage systems currently in use. But, like other forecasters, IDC predicted they will fade into the sunset.

Serial-Attached SCSI (SAS) is the drive interface of choice for high-performance enterprise requirements. SAS-based HDDs represented 41% of enterprise unit shipments last year, and nearline (NL) SAS drives accounted for another 11%, according to IHS iSuppli. NL-SAS drives use the higher-performance SAS interface, although they're generally the mechanical equivalent of enterprise 7,200 rpm SATA disks, giving them the advantage of higher capacity at lower cost.

The SAS interface is also trending upward in the enterprise solid-state market. According to IHS iSuppli, SAS-based SSDs accounted for only 18% of enterprise unit shipments in 2011, but their share rose to 24% last year and could inch up by 1% or 2% per year over the course of the next few years. The eventual transition from 6 Gbps SAS to 12 Gbps SAS should help to address higher performance needs moving forward.

Joseph Unsworth, a research vice president focusing on NAND flash and SSD technology at Stamford, Conn.-based Gartner Inc., predicted that SAS will become the dominant interface for SSDs and HDDs in enterprise storage systems by 2015. However, that SAS trend does not apply to enterprise servers, where PCI Express (PCIe) solid-state storage will dominate, according to Unsworth's projections.

In the meantime, SATA SSDs have fared well, with estimated sales last year of about 4 million enterprise-grade and client/consumer-grade SSDs to enterprise customers, according to Greg Wong, founder and principal analyst at Forward Insights in North York, Ontario. Wong said he mentioned the enterprise- and PC-level shipments because, in many cases, consumer-grade SATA SSDs intended for PCs are actually going into servers, giving users a tremendous performance bump at a relatively cheap price.

"For the price that they're paying -- less than a dollar a gigabyte -- they feel it's sufficient for their needs," Wong said. A potential disadvantage is that the warranty for the consumer drives may not apply to enterprise use, he cautioned.

SATA SSDs have held appeal in particular with Internet infrastructure companies and cloud providers, as well as some small- to medium-sized companies, which purchase them over the Internet, Wong said. He noted that the major cloud providers do considerable testing before putting the multi-level cell (MLC)-based SATA SSDs into production use.

One advantage of the SATA-based SSDs, as with SAS drives, is that they simply plug into the HDD slots of server and storage systems, whether new or legacy, unlike PCIe cards, which generally require a driver and software, Wong said.

Although PCIe and SAS are trending up, SATA remains the dominant interface for SSDs designed for enterprise server and storage systems. IHS iSuppli research shows that SATA SSDs commanded 64% of enterprise unit shipments in 2011. The percentage fell to 56% last year, and IHS projects the share will drop by a few percentage points per year going forward, but SATA still will retain the majority position. In 2016, SATA SSDs will comprise 42% of enterprise unit shipments, with SAS- and PCIe-based SSDs at 32% and 26% respectively, according to IHS iSuppli research.

SATA is also expected to trend downward with enterprise HDDs. IHS iSuppli's statistics indicated that SATA HDDs factored in 47% of enterprise unit shipments last year, and the percentage will slip to 39% by 2016, when SAS HDDs also take 39% and NL-SAS drives reach 21%.

Fang Zhang, a storage analyst at IHS iSuppli, said enterprise users lean toward high-capacity SATA HDDs for data such as video and audio. They buy SAS-based HDDs for applications with medium-performance demands, and they invest in the more expensive SAS- and PCIe-based solid-state technology for their most critical applications with the highest performance requirements, she said.

With SSDs, one of the additional options is NAND flash type. Single-level cell (SLC) drives are the most durable and reliable, capable of 100,000 program/erase cycles (also known as write/erase, or simply write cycles), but they're also the most expensive. Less costly enterprise MLC (eMLC) drives average about 30,000 write cycles, and even cheaper MLC drives handle 10,000 or less, but they became popular once manufacturers mitigated the endurance limitations through overprovisioning, sophisticated controllers, smarter algorithms and other software improvements.

Gartner research showed that SLC represented 84% of enterprise SSD shipments in 2009, but fell to 67% the following year, while enterprise MLC SSD shipments rose to 32%. MLC became the majority in 2012 with 58% as SLC plummeted to 34% of enterprise SSD shipments, according to Gartner. Enterprise MLC also started to gain share, at 8%, Gartner claimed.

According to Gartner, the enterprise SSD mix in 2017 will be 67% MLC, 10% eMLC and 8% SLC, with the up-and-coming triple-level cell (TLC) snaring 15% of the market.

The jury is still out on the use of TLC for enterprise workloads. Also known as three-bits-per cell NAND flash, TLC has the potential to lower prices, but it also carries the baggage of worsening endurance, performance and reliability. Manufacturer Micron Technology Inc., for one, thinks 3D NAND, which stacks memory cells in three dimensions, has a more promising future in the enterprise than the planar TLC technology.

Gartner forecasts show TLC SSDs taking 3% of the enterprise SSD market share this year and 6% next year. Unsworth predicted that enterprises will use TLC for specific read-intensive application workloads, "with the right flash management and data management software features." He said Samsung is shipping TLC SSDs for PCs, and he expects competitors to emerge later this year and in 2014.

"It will gain minor adoption near-term, but ultimately there is opportunity for it with certain customers," Unsworth said. He pointed to hyperscale computing users such as Amazon, Facebook, Google and SSD appliance vendors as potential users of a mix of TLC and MLC SSDs.

Wong said TLC could play out similarly to the consumer-grade MLC SATA drives intended for PCs that are finding their way into the servers of price-sensitive end users. He said he doesn't expect the majority of vendors to push TLC, but some might feel compulsion to try to reduce SSD costs.

"If it's priced cheaper than the MLC client drives out there, there's going to be uptake," Wong said. "Somebody's going to buy it."

Fusion-io CEO David Flynn said MLC makes up about 95% of the PCIe flash cards his company sells today. The rest is SLC, but he can foresee TLC picking up share when the price of TLC comes down more.

"We see use cases where TLC would be more than sufficient from a performance and endurance standpoint," Flynn said. "The supply of TLC has not yet reached a high enough volume to make a significant enough price difference. But that's changing, so TLC could become more mainstream and be 30% less expensive [than MLC]."

Senior news director Dave Raffo contributed to this story.

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