Garbage collection (SSD)

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

Because flash memory must be erased before it can be rewritten, with much coarser 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.

Intel and SiliconSystems (acquired by Western Digital in 2009) used the term write amplification in their papers and publications as early as 2008. Write amplification is typically measured by the ratio of writes committed to the flash memory to the writes coming from the host system. Without compression, write amplification cannot drop below one. Using compression, SandForce has claimed to achieve a typical write amplification of 0.5, with best-case values as low as 0.14 in the SF-2281 controller.

Due to the nature of flash memory's operation, data cannot be directly overwritten as it can in a hard disk drive. When data is first written to an SSD, the cells all start in an erased state so data can be written directly using pages at a time (often 4–8 kilobytes (KB)^[update] in size). The SSD controller on the SSD, which manages the flash memory and interfaces with the host system, uses a logical-to-physical mapping system known as logical block addressing (LBA) and that is part of the flash translation layer (FTL). When new data comes in replacing older data already written, the SSD controller will write the new data in a new location and update the logical mapping to point to the new physical location. The data in the old location is no longer valid, and will need to be erased before the location can be written again.

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Wikipedia