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Low-cost MLC NAND flash gains in enterprise solid-state storage

New specialty NAND flash devices from Fusion-io and enterprise MLC from STEC are seen as solid-state storage alternatives to higher cost SLC, but wide performance gulf remains.

New solid-state drives (SSDs) coming onto the market will challenge the notion that only single-level cell (SLC) flash SSDs are fit for true enterprise applications.

Until recently, the conventional wisdom has been that only SLC has the performance and reliability required for the enterprise, while multi-level cell (MLC) NAND flash is more suited for consumer devices.

However, Fusion-io Inc. is planning a new class of enterprise-class NAND flash that it calls single mode level cell (SMLC) that could give customers faster and lower-cost PCI Express-based solid-state storage to plug directly into their servers. And STEC Inc., which supplies SSDs for most of the major storage array vendors, is sampling what it calls  enterprise-ready MLC drives with capacities up to 800 GB to its OEM partners.

Now it's a question of how much performance organizations are willing to trade for the cheaper solid-state drive alternatives.

"The best way to take the cost out of enterprise SSD is to use MLC chips that are two to three times cheaper than SLC," wrote Joseph Unsworth, research director at Stamford, Conn.-based Gartner Inc., in an email interview. "The caveat is that the quality must be good enough to last in the enterprise SSD environment."

SLC still writes faster, more reliable than MLC

The more economical MLC can store two or more bits per cell and multiple levels of charge, giving it at least twice the capacity of SLC. But with slower write speeds and higher bit error rates, MLC lacks the performance and reliability of SLC.

The industry's oft-used wear-out figures are 100,000 program/erase, or endurance, cycles for SLC flash and 10,000 cycles for MLC flash, although industry sources report the MLC figure actually has been dipping well below 10,000 as the flash dies get smaller.

The 160 GB and 320 GB SMLC ioDrive alternatives that Fusion-io plans to deliver this quarter could mitigate some of the MLC shortcomings. The SMLC product uses MLC flash chips, but Fusion-io claims its more sophisticated controllers deliver write speeds equal to SLC Flash and significant endurance improvements over MLC.

David Flynn, chief technical officer (CTO) at Fusion-io, said while Fusion-io's MLC drives could work in the enterprise, the new SMLC drives will give customers a third option alongside MLC and SLC.

"Customers come in and say 'I need this much capacity, performance and endurance,' and they don't want to pay any more for any one of those than they need to," Flynn said. "Right now, they have a tough decision. Do you buy the more expensive SLC just to get the endurance when all you need is small capacity? Or do you buy the large MLC to get the endurance even though you didn't need that much capacity?"

In some ways, Fusion-io treats MLC like SLC. The SMLC product uses single thresholding, rather than multi-thresholding, in a way designed to extend endurance and performance, Flynn said. Lance Smith, Fusion-io's senior vice president of product marketing, noted that the SMLC offering maps a single bit of data to a single bit per cell, whereas MLC maps multiple bits of data to a single cell.

"We get five times the endurance out of MLC," Smith said.

The slower read speeds of Fusion-io's SMLC match those of its MLC technology, and the price tag falls squarely between MLC and SLC.

STEC claims its OEM customers have shown little interest in the hybrid SLC-MLC model, in which a single drive might include both SLC media for write purposes and MLC media to hold data.

"For those guys that are interested in MLC, they're looking for the dollar-per-gigabyte benefit, and SMLC, having half the capacity [of MLC], makes the cost of it two times," said Scott Stetzer, STEC's director of marketing for SSD programs.

STEC's OEM customers include Dell Inc., EMC Corp., Hewlett-Packard (HP) Co., Hitachi Data Systems, IBM, NetApp Inc. and Sun Microsystems Inc. Its MLC offerings support the same SATA, SAS and Fibre Channel interfaces that its SLC technology does. Although STEC has now launched MLC products aimed at the enterprise, company execs say there will always be a large performance gulf between MLC and SLC.

"The interest, especially recently, is picking up," Stetzer said. "MLC in the enterprise is here [and] here to stay. And it will continue to grow in volumes." But, he added, "For the foreseeable future, SLC will continue to dominate."

Finding a place for MLC in the enterprise

One Fusion-io customer is already putting its MLC device to the test for enterprise use. Inc. had such success as a beta customer of Fusion-io's 160 GB SLC ioDrives for its enterprise resource planning (ERP) databases that it tried its high-transaction storefront/shopping cart applications on solid-state storage last fall. CTO Geoffrey Smalling said the database was too large for Fusion-io's SLC offering, so the online retailer tried the 320 GB MLC option.

Smalling admitted he was nervous after reading blog posts on MLC's 10,000 write/erase, or endurance, cycle limits. But he said Fusion-io's Flynn eased his concerns after explaining the startup's wear leveling algorithm, chip-level redundancy, error correction and system checks.

Smalling said he was especially impressed to hear that each PCIe card has 400 GB of raw capacity, leaving 80 GB free to be substituted when a storage block wears out. Fusion-io also builds in a RAID-like feature called flashback redundancy, which allows a spare flash chip to take over without loss of data in the event of a catastrophic chip failure. can consult Fusion-io's ioManager utility whenever its wants to view the wear-out status of its MLC ioDrives. Smalling said after eight months the chart shows that more than 99% of the MLC drive's capacity remains operational.

Even though Smalling knew MLC's read and write speeds would be slower than SLC, he figured the MLC option's 80,000 IOPS would still provide a "night and day" difference over its prior technology, a lower-end NetApp storage-area network (SAN). An unexpected side benefit was the ability to plug PCIe-based MLC ioDrives directly into its HP ProLiant DL380 G5 servers because neither nor its hosting provider had great SAN expertise.

"To me, the SLC-MLC hasn't been a concern. It was more the transactional redundancy," Smalling said. "We took steps to put in RAID on the servers and mirror the servers." He added that the MLC product's five-year projected lifespan exceeds his anticipated three-year server replacement cycle.

MLC seen as alternative rather than replacement to MLC

Gartner's Unsworth claims while MLC will make inroads in certain segments, he doesn't think it will overtake SLC in the next five years. Mark Peters, an analyst at Milford, Mass.-based Enterprise Strategy Group, added that he expects MLC to remain as an alternative rather than a replacement for enterprise-grade SLC SSDs.

"Generally, the end-user price of SSDs is declining significantly in any case as more vendors enter the market and volumes start to increase," Peters noted.


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