Historically, NAND Flash memory endurance has been calculated by the number of programme erase (P/E) cycles attributed to the type of NAND Flash memory that is deployed within the drive. This would typically represent the total amount of data that an SSD is warranted to be able to write under the specified lifetime. For example SLC NAND 100k P/E cycles, MLC 3k and so on.
However, manufacturers have now moved away from P/E cycles to make things easier for the end user. The endurance is often written on product specification sheets either as Drive Writes per Day (DWPD) or Terabytes Written (TBW), the latter of which is considered by many manufacturers as the preferred explanation. This gives the user a far more accurate measurement of how much data can be written to the device. It's about selecting the right solution rather than looking at what NAND is being used within the device.
TBW represents the total amount of data written to the SSD over the specified lifetime. In enterprise and data centre environments, where applications and workloads are known, DWPD tends to be the preference and relates to how many times an SSD’s useable capacity can be overwritten in 24 hours within the specified lifetime or warranty period. In client and industrial applications where the workloads vary, TBW is typically used to determine SSD endurance.
Converting DWPD to TBW and vice versa
As long as the SSD capacity and specified lifetime is available, converting from DWPD to TBW, and vice versa, is straightforward:
TBW = DWPD x SSD capacity x Lifetime (warranty years) x 365 days
TBW to DWPD
DWPD = TBW (SSD capacity in TB) x Lifetime (warranty years) x 365 days
NAND Flash dependant
Different types of NAND Flash, SLC, MLC, pSLC, 3D TLC, and QLC all have different performance characteristics, capacities, endurance, cost and reliability. Finding the correct endurance level for your application is a bit of an art, so seeking specialist advice is certainly worthwhile.
Whether the drive is DWPD or TBW rated, SSD endurance is dependent in part on the NAND flash memory endurance. Each time that data is written to NAND flash memory cells, one Program/Erase (P/E) cycle is generated and the cell degrades ever so slightly. Read more about NAND Flash technology here
How does NAND Flash wear out?
As more P/E cycles are generated, the oxide layer surrounding the NAND weakens and cannot hold the electrical charge required for storing data. SLC NAND (1-bit per cell) is less impacted by each P/E cycle versus MLC, TLC and QLC NAND which stores multiple bits per cell. So, in time as more and more data is written and erased, the cells will degenerate and the device will eventually fail. This is probably the Achilles heal for SSDs, but you will have to be writing vast amounts of data for this to happen.
MTBF – Mean time Between Failures
MTBF or mean time between failures is calculated as the average period of time that the storage device is operational until it fails. It is expressed in hours, and the higher the number, the more reliable the storage device is. Industrial SSDs are rated for millions of hours, which could translate to hundreds of years. An MTBF of 2,000,000 for example, means that the SSD can operate reliably for over 228 years running 24 hours a day.
Seek expert advice
Selecting the right SSD with the right endurance levels for an application can seem confusing with all the different options available from hundreds of manufacturers. Coupled with customisation, which could enhance the performance and endurance of the SSD, understanding this technology requires specialist advice.