Write amplification 写入放大

Write amplification (WA) is an undesirable phenomenon associated with flash memory and solid-state drives (SSDs) where the actual amount of physical information written is a multiple of the logical amount intended to be written.

Because flash memory must be erased before it can be rewritten, with much lower granularity of the erase operation when compared to the write operation, the process to perform these operations results in moving (or rewriting) user data and metadata more than once. Thus, rewriting some data requires an already used portion of flash to be read, updated and written to a new location, together with initially erasing the new location if it was previously used at some point in time; due to the way flash works, much larger portions of flash must be erased and rewritten than actually required by the amount of new data. This multiplying effect increases the number of writes required over the life of the SSD which shortens the time it can reliably operate. The increased writes also consume bandwidth to the flash memory which mainly reduces random write performance to the SSD. Many factors will affect the write amplification of an SSD; some can be controlled by the user and some are a direct result of the data written to and usage of the SSD.