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.

WhipTail: Software solves MLC SSD issues

July 29, 2009 – One of the more promising developments in the solid-state disk (SSD) drive space is the potential use of low-cost multi-level cell (MLC) NAND flash memory in enterprise applications and arrays, vs. the high-cost – but more reliable and durable – single-level cell (SLC) technology, as I mentioned in my previous post (see “Intel slashes SSD prices” ). This can basically be accomplished in two ways: via software or via controller enhancements.

Relative newcomer WhipTail Technologies is an example of a vendor that’s using software techniques to overcome some of the inherent limitations of MLC flash memory; namely, write amplification issues that limit the ability of NAND to perform random writes in an effective manner (a performance issue), and wear-out issues (a reliability, or durability, or endurance problem).

To address the performance part of the equation, WhipTail uses buffering (not caching) techniques, in which writes are aggregated into a buffer that’s sized to the erase block of the NAND, according to WhipTail CTO James Candelaria, who claims that this technique enables performance close to the performance specs of the NAND media.

Specifically, the company claims performance of more than 100,000 I/Os per second (IOPS) of sustained, random I/O with 4KB block sizes and a 70/30 read/write split. Other performance specs include a latency of 0.1 milliseconds, and bandwidth of 1.7GBps (internal to the chassis).

The other major problem with MLC flash is wear-out. For example, SLC is rated at about 100,000 cycles per cell, while MLC is rated at only 10,000 cycles/cell before the cell becomes unreliable (and that may go down to 2,000 to 4,000 writes/cell with smaller die sizes).

To address the wear-out issue, WhipTail uses a technique called linearization, which essentially entails writing forward across the disk and not revisiting blocks until the entire array has been utilized. This not only decreases wear on the media, but also increases performance. Working in conjunction with linearization, a defrag process ensures that there is always a minimum amount of free space. This technique also works in conjunction with the drive’s wear-leveling algorithms.

The company’s internal tests indicate that if you rewrite an entire array once a day, the device will last seven years (or longer than most other components in the storage hierarchy).

You can get the details on these two techniques on WhipTail’s web site, as well as details on their products, but what about pricing?

Candelaria contends that WhipTail “provides tier-0 [SSD] performance at the price of tier-1 arrays.”

Well, a 1.5TB WhipTail SSD array is priced at $46,000 retail; a 3TB version at $75,600; and the new 6TB configuration, introduced this month, at $122,500.

Summit NJ-based WhipTail was spun out of TheAdmins, a reseller, early this year and has been working on its SSD technology since late 2007. Its first product went GA in February. The company sells through resellers, with eight VARs signed up so far.

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

And if you’re really interested in solid-state technology, consider attending the Flash Memory Summit, August 11—13 at the Santa Clara Convention Center.