A file system issues TRIM commands as a result of deleting a file to tell the ssd that the set of blocks which previously made up the file are no longer being used.  The sdd can then mark those blocks as 'free'.  Later when the ssd internal garbage collection runs, then it knows that it doesn't have to preserve the contents of those blocks.  This makes garbage collection more efficient.  There are lots of articles that explain this.   The trouble this causes for parity-based array organizatio