SSD gets slower as they age

SSD becomes slower as they age. This is not my experience because I do not own SSD, but I have heard people complain. Windows Internals (edition 6) by Mark E. Russinovich explains why performance of SSD goes down.

Sector, Block and Page in SSD
     Sector/Page = 4,096 byte.             // (Unit of Read/Write)
     Block = 1024 * sector                  // (Unit of Erasing)
     Chip  = Thousands of Blocks

Unit of Read/Write and Unit of Erasing
Key thing is Unit of Erasing is larger than Unit of Read/Write. This affects the speed at which data is written to blank SSD and crowded SSD. Writing to blank SSD is fast because there is no need to make space, and writing to crowded SSD is slow because making space is an expensive operation.

Example, here we write a single sector on to a crowded SSD
1.   Read the entire Block into the controller’s internal RAM. (because unit of Erasing is Block)
2.   Erase the Block in the SSD.
3.   Write new sector in to Block sitting in RAM.
4.   Write the entire Block on to the SSD
As you see, writing one sector required working on extra 1023 sectors because unit of Erasing (one Block has 1024 sectors) is Block. So as SSD gets crowded, the rate of erasing increases resulting in not only bad performance but also aging because SSD has fixed number Write cycles.

Source: Windows Internals (edition 6) by Mark E. Russinovich

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