Eric Shanks of theITHollow comments:
Because modifying a page, or deleting a page takes time, the Operating System usually updates the file system to say that the page is available to be overwritten, but never informs the disk controller. Since the disk controller never knows what pages have real data in them, and which pages are invalid, they are never deleted. Normally, this speeds up the process because you don’t have to go through a whole routine of deleting the actual data. The OS shows the amount of “Free Space” which is = (Total Unwritten Pages) + (Total Invalid Pages) * (Page Size).
SSD may be fast, but there are some things that can cause it to start running much slower than expected. Read on for an explanation of why that is along with some fixes to keep it from happening.
Read more at: Understanding the SSD write performance cliff