A common concern with the current crop of Solid State Drives is the performance penalty associated with block-rewriting. The flash memory used on today's SSDs is comprised of cells that usually contain 4KB pages that are arranged in blocks of 512KB. When a cell is unused, data can be written to it relatively quickly. But if a cell already contains some data--no matter how little, even if it fills only a single page in the block--the entire block must be re-written. That means, whatever data is already present in the block must be read, then it must be combined or replaced, etc. with the new additional data, and the entire block is then re-written. As you can surmise, this process takes much longer than simply writing data straight to an empty block.This isn't a concern on fresh, new solid state drives, but over time, as files are written, moved, deleted, replaced, etc., many blocks are a left holding what is essentially orphaned or garbage data and their long-term performance degrades because of it.To mitigate this problem, virtually all SSD manufacturers have incorporated, or soon will incorporate, garbage collection schemes into their drives' firmware that actively seek out and remove the garbage data.
