![]() Many of today's consumer-grade SSDs use 19-21nm NAND where each block is good for about 3,000 cycles before becoming unusable, although enterprise SSDs and several high-end consumer SSDs that use more durable types of NAND are commercially available. SSDs try to spread writes evenly over the NAND to avoid premature failure, a process which relies on the free space available to the drive.Įach NAND block can only sustain a finite number of write/erase cycles. The SSD is then free to garbage-collect those unused blocks. In order for the SSD to know which blocks can be erased, the operating system must tell it which blocks no longer contain valid data. This cleanup process is called garbage collection. At a more appropriate time, ideally when the drive is idle and all pages in a block are marked invalid, the SSD can erase blocks that are no longer in use. This means that whenever data is rewritten, the SSD must mark the data in the affected pages invalid and rewrite it elsewhere, possibly in a different block. Source – Courtesy Music Sorter at Wikipedia, CC BY-SA 3.0Įach page may be written to individually, but pages cannot be rewritten until erased, and erasing can only be done in whole blocks. The following diagram assumes 256 KB blocks, but the concept is the same regardless of the block size. Most modern SSDs uses NAND with blocks of 128 pages for a block size of 512 KB, although some drives, especially older ones, may use 256 KB or smaller blocks. NAND flash is divided into blocks, each consisting of a set of pages typically 4 KB in size (plus error-correcting codes).To understand why this is the case, it is necessary to explain how SSDs work on the inside. Repeatedly overwriting a file before deleting it will not securely erase it on an SSD-the data would just be written elsewhere on the NAND. ![]() Because of the nature of NAND flash memory, SSDs cannot directly overwrite data.
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