Driving Out MLCs' Weaknesses

Arthur Cole

In SSD circles, the debate between single-level cell (SLC) and multi-level cell (MLC) technology is ongoing. SLC provides higher throughput, which is the primary driver for solid-state disks in the enterprise, but MLC provides lower cost per GB.

But recent developments in MLC technology are starting to eat away at SLC's advantages, even as they bring the cost of enterprise-grade Flash memory down. Some of these are on the Flash structure itself, and some are on the controllers and interfaces used to connect to the wider enterprise environment.

A case in point is Pliant Technology's new Lightning devices. The line is based on consumer-grade MLC Flash, but is outfitted with a new controller and software that address some of the wear-leveling and storage-management issues that have hampered MLC systems so far. As for overall performance, the company's LB series is rated at 10,000 IOPS and is available up to 400 GB at a list price of about $10 per GB. You also get a native SCSI interface.

Over at STEC Technologies, the focus is on advanced signal processing and data management to improve MLC performance. The company's CellCare and SAFE (Secure Array of Flash Element) systems are geared to boosting write endurance and data reliability, with an eye toward performance parity with SLC systems for at least a five-year lifespan. The company also claims to have virtually eliminated data-integrity issues.

There is also growing support for the new smaller-form MLC designs on the drawing board. Fusion-io just came out with a new ioMemory module that supports 3x nm MLC for PCIe environments. The device will allow standard PCIe cards to pack a whopping 1.28 TB of memory, while delivering an impressing 285,000 sustained IOPS.

The economics of supply and demand being what they are, there is a good chance that MLC will become more popular as prices continue to fall. Following the introduction of enterprise-class SSDs, increased demand drove MLC storage to more than $2 per GB, according to market watcher Isuppli. Nowaday's the technology is on track to drop to about $1 per GB by the end of the year, driven largely by advances in 3-bit-per-cell technology as opposed to the more common 2-bit MLC systems.

At the moment, however, very little 3-bit Flash has made it into enterprise-class devices. But that may be changing. Intel and Micron are now sampling a 3-bit system, delivering 64 Gb on a 25 nm process, about 20 percent smaller than current MLC designs. Although it is aimed largely at Flash cards and thumb drives, it doesn't take a big stretch of the imagination to see the companies coming out with an enterprise system soon.

With all this activity, the question for most IT managers is whether it makes sense to invest in multi-tier Flash arrays, with SLC handling the high-traffic, high-speed data and MLC stepping in when more capacity is needed. Such a scheme already requires a high degree of data management, particularly when there are already multiple tiers of hard disk and even tape storage in the mix.

But until full parity is reached between the two platforms, multi-tiered Flash arrays will remain in play. But who knows? Perhaps a few more years of development will foster a whole new round of storage consolidation, this time on the SSD level.

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