Using Real-World Methodologies to Calculate SSD Usable Life

With more embedded systems using SSDs in critical applications, designers are now asking the question, “How long will this SSD last in my application?” To help answer this pressing question, it is important to review the recent changes in NAND flash technology.

NAND flash components are the primary storage media in SSDs, and are experiencing technology changes at exponential rates. The quests for lower cost per bit and smaller size requirements are driving NAND flash technology to go to smaller process geometries and store multiple bits per cell, introducing new challenges for reliability and product longevity.

The reliability concern relative to NAND flash-based SSDs is centered on the limitation to the number of write/erase cycles of the device. Embedded system OEMs wonder if an SSD will meet their long-term deployment requirements in 24/7 applications with intensive write/erase usage. The fact is, SSDs are inherently more reliable than hard drives.

Write Amplification

NAND flash must be managed. The SSD controller manages endurance by using wear-leveling and other storage management algorithms to optimize write/erase operations to increase endurance at a system level. In addition, SSD controllers reserve a “spare area” in the NAND flash array to mange bad blocks and other flash vulnerabilities. The number of spares in an SSD is 1 to 2%, but it can be as high as 50% in applications that require high reliability. This technology, called over-provisioning, is accomplished by providing additional NAND capacity specifically to address reliability issues.

Important to accurately calculating SSD useable life is the concept of write amplification, which defines the minimum number of writes the controller makes to the media for every write command from the host system. Write amplification highlights the fundamental mismatch between erase block sizes and page sizes. For example, the minimum write size for an SSD controller may be a 4 kilobyte (KB) page size. Most SSDs must erase before writing, which can require that a whole erase block (256KB) be erased and written. The resulting write amplification of this example would be 256:4 or 64:1. The worst-case scenario is writing to the same logical block address over and over again, which would result in the 64:1 ratio. The best-case scenario is streaming data in file sizes that are integer multiples of the erase block size. In this case, the write amplification would be 1:1. In practice, the write amplification is somewhere in the middle based on how the host writes the data. This illustrates that the usage model can have a 64× impact on the usable life of the SSD.