WAF

WAF – Write Amplification Factor – indicates the extra NAND media write operations the SSD performs in response to the host writes (WAF = total NAND media writes / total host issued writes). The additional media writes are necessitated due to the way in which NAND flash media handles writes. NAND flash pages once programmed cannot be overwritten, unless the entire flash block (1 block = N pages) is erased again. Since the erase is a costly operation, the SSD firmware avoids the unnecessary erase operations by handling the writes in a log structured manner. So any overwrites will be redirected to a new flash page and the old page is marked as invalid. Eventually many such invalid pages are generated. The Garbage Collection (GC) process of SSD will handle such pages by moving the valid pages to a new flash block. GC process also releases these flash blocks for erase and eventually for new writes. The GC causes additional writes due to the movement of valid pages, in addition to the ongoing host initiated writes. This process is the root cause of WAF problem in SSDs. The extent of the WAF problem can vary based on the active host workloads. For example, a sequential workload might not cause much of WAF since it aligns with the log structured write design of the SSD software, while a random workload with lots of overwrites can cause high WAF in SSD. [Note from Samsung Tech Blog.]