SanDisk unveils durability metric for solid-state drives

Implications: * SanDisk recently revealed a proposal for creating a single metric representing the expected lifetime of a solid state drive  * The SanDisk metric is called “Longterm Data Endurance” or LDE  * The SanDisk proposal comes at a time that several organizations such as IDEMA and JEDEC are discussing standards for SSD reliability  * SNIA, the Storage Networking Industry Association, is also launching a solid state initiative that will explore the evolving use of solid state storage  * These are important developments for sustainable growth of SSDs in the enterprise and computer markets

Analysis: Sandisk released details on its proposed standard metric for solid state drive (SSD) expected lifetime.  This metric is called Longterm Data Endurance (LDE).  SSDs are subject to wear at the flash memory cells that limit the number of times that these cells can be written.  Flash memory devices such as SSDs deal with this wear using special algorithms that run in the flash memory controller, called wear leveling.  Wear leveling spreads the wear around the flash memory cells to prolong the life of the memory cells.  Wear leveling is even more important for multilevel cell flash where multiple bits are stored on each cell since this sort of flash memory cells wear much faster.  With the proper wear leveling and excess capacity for memory management and space sectors flash memory can be made to last for quite a while, depending upon the usage.

At about the same time that SanDisk introduced this proposed standard the IDEMA reliability group that created standards for HDD reliability has begun to work on creating new standards for SSD reliability.  Similarly JEDEC, another standards group that focuses on solid state memory technology has begun an effort to create metrics for endurance, reliability, performance and data lifetime for flash memory devices.  SNIA, the Storage Networking Industry Association is initiating a flash memory initiative of its own that looks at new uses for solid state storage in storage system architecture.  Clearly there is much work going on in this area.

It is good that standards bodies are starting to look at creating verifiable standards for SSDs.  There has been much hype over the last few years on SSD performance and reliability and many people have found that the actual increase in performance and reliability are no better than many hard disk drives despite the higher prices.  The results conflict based partially upon the test methodology and measurement methods used, so clearly there is need for agreed upon measurement methods and reliability metrics.  This is what standard organizations can create.  On the other hand it needs to be determined what each of these various groups will be doing regarding SSD standards.  Competing standards will not clarify the situation for designers or users.

Moving from relatively well understood chip level reliability metrics to solid state drive reliability metrics is complicated by the various features and algorithms running on various flash memory controllers that implement wear leveling and other flash memory controller functions.  The LDE metric proposed by SanDisk measures the total number of Terabytes a solid state drive can write in its life.  The standard is based in part on Bapco standards for computer performance metrics.

It remains to be seen whether the SanDisk endurance metric will be used throughout the industry but there is a clear need for the various potential standards groups to work together to create a meaningful SSD reliability metric.  This metric should be capable of comparing various SSDs as well as the endurance of SSDS compared to HDDs.