The big slowdown
Slowing down - why?Then there's the problem of how SSDs perform read and writes. Remember the cells from MLC and SLC? They're the building blocks of the NAND storage. Cells are grouped together into pages, and pages into fixed-size blocks. Typically, a page is 4K in size and it can be read and written to in the normal fashion. This is how the drive ships when you buy it and benchmark scores usually reflect the 'new' drive's characteristics.
How do potential problems arise? First off, it's important to note that individual pages cannot be deleted on the SSD; you have to delete the entire block, and that's inherently how flash-based memory works. Secondly, the SSD's controller is never informed by the OS once a file deletion is requested. Should you wish to write more data to the drive, the OS, as far as it's concerned, believes that the previously-deleted space is free on the SSD. The SSD, oblivious to the original delete command, knows that the page isn't actually free and must be physically deleted if it's going to tally-up with the OS. The situation (and speed) is kept in check until the drive is rammed with more data, to the point where the mismatch between SSD controller and OS, pertaining to free space, hits the limit of the drive.
Therefore, as the drive becomes fuller and fuller, the 'deleted' pages need to be removed by the controller. Easy, eh? Trouble is, as noted above, flash memory cannot delete individual pages. Rather, the block that they belong to must be deleted and rewritten if you want to use the same page, and the contents of that block have to be exported out to the on-drive cache, if provided, or memory and read/written again - write amplification, if you will.
Think about it some more and the net effect is a slowing down of the SSD's performance, as more writes are taking place, for the same request, than when the drive is in a 'fresh' state, with lots of unused space. The situation is worst when writing 4K data to a nearly-full drive, where the entire block needs to be spat out again, and it's here where write amplification is the most noisome with respect to performance.
