Write amplification factor (WAF) is a numerical value that represents the amount of data a solid state storage controller has to write in relation to the amount of data that the host’s flash controller has to write. The numerical value is calculated as a rate by dividing the amount of data written to flash media by the amount of data written by the host.
A write amplification factor of 1 means that 1GB of data was written to the flash host media and the host’s flash controller also wrote 1GB. A write factor of 1 is ideal, but occurs less frequently as the media gets used and the number of program/erase (P/E) cycles increases. Program/erase cycles are managed by the device's flash controller, which uses a logical to physical mapping system known as logical block addressing (LBA) to manage data. When data is rewritten, the flash controller writes the new data in a different location and then updates the LBA with the new location. Invalid data continues to reside in the old location and those cells must be erased before they can be written to again. Because each cell can only tolerate a finite number of program/erase cycles before becoming unreliable, the higher the write amplification factor, the closer the media is to end-of-life.