Hu Yoshida

What is the difference between MLC Flash and eMLC Flash, and is it required for Enterprise Flash?

Blog Post created by Hu Yoshida Employee on Nov 7, 2014

Feb 3, 2014

 

One of my top 10 IT trends for 2014 called for the explosion of enterprise flash. Now you are seeing more and more vendors introducing flash arrays that claim enterprise performance and durability based on something called eMLC, where the “e” stands for enterprise and MLC stand for Multi Level Cell or 2 bits per cell.

What is eMLC and what has been done to create an MLC cell that makes it more durable than the 2000 to 3000 P/E cycles that are associated with MLC? The engineers tell me that there is no difference in the silicon. Some manufacturers sell an eMLC, which has better durability than MLC. Since MLC has multiple voltage levels, four states to distinguish the 2 bits in a cell, eMLC slows down the programing time (tProg) to write the states and erase more carefully so that the cell can last longer before it needs to be refreshed. They may also cherry pick the wafers to maximize the number of good cells. This adds to the cost of eMLC over MLC but the basic technology is the same. While this is done at the flash cell level, more needs be done at the flash controller level to ensure enterprise performance and durability for flash.

Chris Evans explains some of the features that go into making flash ready for enterprise use in an interview withComputer WeeklyIn my post, I explained some of the enterprise features that Hitachi has developed for the controller in the Hitachi Accelerated Flash module. The list of features is very similar. (I also explained how we eliminated the Write Cliff, which impacts QoS, and increased performance and flash density.) Notice that these enterprise features are provided by the flash controller regardless of whether the cell technology is eMLC or MLC. The use of eMLC may even degrade performance due to the increase in tProg. Since flash requires a lot of programming cycles, the power of the processor in the controller determines the capacity and performance of the flash module.

So when vendors say that they are using eMLC to create enterprise flash storage, check to see if they are also using a flash controller with enough processing power to support features like wear leveling and extended ECC, reduce write amplification, and eliminate the write cliff, and increase capacity and performance over commodity flash modules or SSDs. In other words, an enterprise SSD or flash module must not only mask the P/E limitations of MLC technology but also provides the performance, capacity and QoS that is needed for enterprise storage applications. Instead of focusing on eMLC, they should be focusing on eSSD to differentiate this SSD with an enterprise class controller function from SSDs with commodity controllers. Hitachi’s Accelerated Flash Module, HAF, can also be classified as an eSSD.

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