MLC for enterprise SSDs: The controller angle

August 3, 2009 -- According to conventional wisdom, enterprise-class SSDs require single-level cell (SLC) technology, as opposed to the lower-cost multi-level cell (MLC) technology – both of which fall under the NAND flash memory category. That’s because, generally speaking, SLC has performance, reliability and endurance (longevity) advantages. However, SLC media is much more expensive than MLC media. (In terms of raw media, the cost difference is about 4x.) As such, a variety of vendors are working on ways to combine the advantages of SLC with the low cost of MLC, in many cases using MLC media with advanced software and/or hardware.

In my last post (see “WhipTail: Software solves MLC SSD issues” ) I looked at WhipTail Technologies’ software-based approach to this issue. Other SSD vendors, such as STEC, are addressing the issue primarily from the controller angle (which, of course, is very much a software issue as it involves firmware and algorithms).

I recently spoke to Scott Shadley, senior product manager at STEC. He identifies the following as the key areas that will require advancements if MLC is going to succeed in enterprise-class SSDs.

--Existing ECC algorithms in SSD devices need to be improved, because ECC requirements for MLC are much more stringent than for SLC.

--Vendors need to mitigate the wear issues with MLC, because there’s about a 10x difference in the number of recycles per data sheet that SLC can withstand vs. MLC. Many SSD vendors utilize wear-leveling techniques to mitigate this problem, but “the goal is to limit, or minimize, writes in general, which wear leveling does not address,” says Shadley.

“Vendors have to come up with new ways for the controller to handle, or manipulate, incoming data so that the SSD is only writing a minimal amount of times, either by controller caching, external caching, and/or algorithms that manipulate the data to minimize the amount of data stored on the media,” he explains.

--Shadley also notes that, because MLC is slower than SLC, enterprise-class SSDs based on MLC technology will require faster processors, more flash channels, better ways of accessing the flash on those channels, and the ability to take advantage of the new NAND interfaces. “The goal is to minimize the performance and cost differences between MLC and SLC,” he says.

I expect to see some announcements in this space at next week’s Flash Memory Summit, August 11—13 at the Santa Clara Convention Center.

For more information on SSDs, see InfoStor’s SSD Topic Center.