For this the TRIM command was established. Another approach is to tell the operating system to forward the delete command. For example it writes to empty cells first and in the background it optimises the memory structure. On the one hand the SSD controller can tend to the problem through various strategies. In order to avoid this, there are different strategies. Therefore the longer you have already used the drive, the more write-access actions have to overwrite data. Quite bad for SSDs as this takes very long. Therefore the operating systems pass no delete commands for data on to the drive. As HDDs can only write data, there is no deletion available on them. Well you might think that overwriting should not occur that often - unfortunately this is false. Therefore a 4KB writing action leads to a long chain of commands. If you only want to overwrite a separate 4KB area, the controller has to read out 512 KB (1 Block) to change the data in the memory and then delete the block and re-write the shifted data. And this is where the biggest performance problem for SSDs lies. 128 pages (512 KB) are then once again organised in blocks. This is the smallest unit that can be written and read. The individual memory cells are now organised in pages = 4KB. In this review, we focus only on SSD drives with MLC chips, as SLC SSDs are still not mass-marketed due to higher prices. Curent MLC chips are good for 10,000 write-/delete-cycles AND SLC for 100,000 write-/delete-cycles. Another key point of SSD drives is the durability. However in operation the double can be read/written to. Therefore twice as much data fit on the chips, however the average read- (2x) and write-speeds (3x) fall per NAND cell. MLC saves 2 bits (00, 01, 10 or 11) per transistor. SLC chips save only 1 bit (0 or 1) per transistor and for this reason can be read and written to more quickly. SLC (Single-Level Cell) NAND memory and MLC (Multi-Level Cell) NAND memory. There are two kinds of memory chips which are deployed in current SSDs. The abbreviation SSD stands for Solid State Disk and constitutes a drive which is built from non-volatile memory chips (and not out of magnetic disks as in HDDs).
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