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Solid-state drive manufacturers use a technique called SSD overprovisioning to improve the performance and lifespan...
of their drives. Overprovisioning reserves a certain portion of the space on an SSD for use solely by the disk controller. This space is completely invisible to the operating system.
Typically, the average SSD will be approximately 7% overprovisioned. In some cases, it is possible to reserve additional space for the controller's exclusive use, but you cannot reduce the level of overprovisioning that has been put into place by a disk manufacturer.
Because overprovisioning decreases the amount of usable space on SSDs -- which are known for having a high cost per gigabyte of storage -- it is easy to question why anyone would want to sacrifice additional storage space. The primary goal of SSD overprovisioning is to improve the performance of write operations.
One thing that makes SSDs different from other types of disks is that existing data must first be erased before a storage block can be overwritten. Unfortunately, the array's process tends to be somewhat slow. Storage blocks are divided into a series of pages, some of which may contain data that is still in use. Before a storage block can be erased, any pages that contain valid data must be moved elsewhere. This move operation, and the subsequent erasing of the data within the storage block, can take time, which is why SSDs tend to get slower as they fill up.
The space reserved for the disk controller by SSD overprovisioning can function somewhat similarly to a write cache. Rather than taking the time to reallocate storage pages and array storage blocks during a write operation, overprovisioned space can be used to temporarily accommodate the new data until the garbage collection process eventually erases the contents of unused storage blocks.
Some manufacturers also use SSD overprovisioning as a tool to extend the life of a drive. If, for example, a storage block goes bad, then the storage block could potentially be remapped to available space within the overprovisioned area.
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