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Can MLC Endure Your Data Load?

Can MLC Endure Your Data Load?
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Since the early days of SSDs, the drives have featured one of two NAND flash memory types: single-layer cell (SLC) and multi-layer cell (MLC). Enterprises almost universally adopted the former. Because MLC could store multiple data bits in each NAND cell, the technology offered higher storage capacities at lower price points. However, SLC memory was both significantly faster and, more importantly for business applications, offered far higher endurance.

In the SSD world, endurance is the measure of how many write (program/erase) cycles a NAND cell can sustain before failing. Failure can mean different things, depending on the context, but in general it breaks down to an inability for NAND cells to retain data in the absence of power, failure to stay within a given tolerable error range, or inability to maintain a given capacity. For this last point, keep in mind that NAND cells inevitably fail even early on in an SSD’s life. It’s statistically unavoidable, and drives provision a given amount of excess capacity in order to flag bad cells and retire them without impacting addressable capacity. When addressable capacity begins to decline because of cell death, though, we can call that drive failure.

This is an approximate description of SSD failure and endurance. For the actual standards-based definition, refer to the JEDEC Solid State Technology Association’s Standard JESD218, “Solid-State Drive (SSD) Requirements and Endurance Test Method.” JESD218 marks the first if not only time a computing industry standards body has defined endurance expectations for enterprise-grade SSDs. Note that, as mentioned above, some failure is inescapable on a cellular basis. The question is whether or not a storage product at the drive level will endure within a given time frame and JESD218’s criteria.

More to the point, if an MLC-based SSD can meet JESD218’s demands, then one can rest assured that it may well be a dependable contender against SLC in the data center. But how exactly do SSD cells die? And is MLC facing a new threat in the future even as some of its current drives meet JEDEC’s requirements?