Mark is, as usual, absolutely correct on this. I just thought I'd emphasise this point: the layout of blocks that the OS sees bears little relation to the layout of blocks in flash.

For example, if the OS writes to (what it thinks is) block #12345, which is currently mapped (by the SSD controller) to #98765, and the SSD controller decides it needs some wear leveling, it might move that data to block #50943. The OS still thinks the data's on block #12345, in its view of the world, but it's been moved in flash.

Spinning iron disks actually do something similar for bad sector remapping, and the OS (usually) doesn't know anything about that either. This is not new.

What it means is that what looks like a nicely-defragmented file in the OS's linear view of the disk might actually be scattered all over the flash blocks or physical sectors.