MLC vs. eMLC – What’s the Difference?

From time to time IT managers ask The SSD Guy if there’s an easy way to compare SSDs made with MLC flash against those made using eMLC flash.  Most folks understand that eMLC flash is a less costly alternative to SLC flash, both of which provide longer wear than standard MLC flash, but not everyone realizes that eMLC’s superior endurance comes at the cost of slower write speed.  By writing to the flash more gently the technology can be made to last considerably longer.

So how do you compare the two?  OCZ introduced MLC and eMLC versions of the same SSD this week, and this provides a beautiful opportunity to explore the difference.

As you would expect, the read parameters are all identical.  This stands to reason, since the only difference between MLC and eMLC flash chips is the way they are programmed.  The write parameters in the table below show just how different these two technologies are:

(DWPD, drive writes per day, is a measure of the durability of the drive.)

The column labeled “Difference” is the one to really consider.  The OCZ Z-Drive 6000 uses MLC flash, while the Z-Drive 6300 uses eMLC, and is specified to provide three times the endurance of its standard MLC counterpart, but at slower speeds.  4K write IOPS are only 75% of the write IOPS of the MLC version, and the sequential write speed is 74% as fast.

The 70/30 read/write IOPS number represents something similar to a standard workload, so it has a lot of reads.  Since the read speed of the eMLC SSD is equal to that of the MLC SSD then the speed gap for this test is smaller: The eMLC SSD is 85% as fast as the MLC SSD.

When it comes to write latency, the MLC SSD has only 83% of the latency of the eMLC version.  Both versions of this SSD have the same read latency.

So, in brief, while eMLC will provide you with three times the endurance of MLC in this SSD, you get 15-25% lower performance.  It’s up to you to decide if that is a worthwhile trade-off.