Write endurance is the number of program/erase (P/E cycles) that can be applied to a block of flash memory before the storage media becomes unreliable.
NAND flash wears out because the oxide layer that isolates the floating gates in a flash memory chip are compromised after repeated P/E cycles. The floating gates are important because they store the electrical charge that indicates the state of each bit in a flash memory cell. For instance, in single level cell flash (SLC), each bit is a “0” or a “1.” When the insulating layer has sustained enough damage, the bit cells may not retain the right charge. “1s” may become “0s” and the data on the storage media becomes corrupt.
Write endurance is calculated by estimating how often and how thoroughly the flash media is used. Some manufacturers provide an estimate of longevity based on an arbitrary amount of data written/erased per day but many vendors simply provide a mean-time-to-failure (MTTF) estimate without stipulating how they have arrived at estimate. SanDisk has proposed an industry metric for write endurance called Longterm Data Endurance (LDE). The metric, which sets a data retention period of one year, is based on a pre-determined usage pattern, assuming the data is written equally over the lifetime of the drive.