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Although flash memory allows fast random writes in small (usually 512 byte) blocks, its distinctive feature is that every block must be erased before being written to. But erasing is slow compared to reading and writing, so manufacturers design memory chips so that they always erase a large group of blocks at once (which takes the same time as erasing one block could take). This group of blocks is called «erase unit» is typically 2-4 megabytes in size. Another distinctive features is that the total number of erase/program cycles is physically limited — after several thousands cycles (a usual number for MLC memory) the block becomes faulty and either just stops accepting new writes or loses the data previously written to it. Denser and cheaper (MLC/TLC/QLC, 2/3/4 bits per cell) memory chips have smaller erase limits, while sparser and more expensive ones (SLC, 1 bit per cell) limits can be as large as 100000 rewrites. However, all limits are still finite, so stupidly overwriting the same block would be very slow and would break SSD very rapidly.
But that’s not the case with modern SSDs, they are very fast and last very long. Even cheap models are usually rather strong. But why? The credit goes to SSD controllers: SSDs contain very smart and powerful controllers, they usually have with at least 4 cores and frequency of 1-2 GHzclock frequency, which means they’re as powerful as mobile phones' processors. All that power is required to make FTL firmware run smoothly. FTL stands for «Flash Translation Layer» and it is the firmware responsible for translating addresses of small blocks into physical addresses on flash memory chips. Every write request is always put into the space freed in advance, and FTL just remembers the new physical location of the data. This make writes very fast. FTL also defragments free space and moves blocks around to achieve uniform wear across all memory cells. This feature is called Wear Leveling. SSDs also usually have some extra physical space reserved to add even more endurance and to make wear leveling easier; this is called overprovisioning. Pricier server SSDs have a lot of space overprovisioned, for example, Micron 5100 Max has extra 60 % of the user capacity on top of it.
Именно из наличия FTL вытекает и проблема с энергонезависимостью и «power loss protection»-ом. Карты отображения секторов — это метаданные, которые при сбросе кэша тоже нужно сбрасывать в постоянную память, и именно этот сброс и вносит торможение в работу настольных SSD с fsync.
