Question on housekeeping of blocks ...

I'm trying to understand how the SSD housekeeping works. I think I understand TRIM and the new NTFS housekeeping feature, but using the SSD on a Mac with OS X I understand it doesn't apply.

However, I'm wondering if a disk which has plenty of space remaining (let's say at least 25-50%) will eventually run into a slowdown. My understanding is that the disk doesn't understand the ext.journaled file system so it doesn't understand what blocks are actually free.

However when a block gets overwritten, it's my understanding that an SSD will route the write to a fresh block (based on least usage to balance the wear out on the chips) so in those cases it should be able to know that the old block is free.

So, my question is this: With overwritten blocks is there some kind of housekeeping and clearing when the disk is idle/rebooting or are those known blocks still lingering in the queue until there are no fresh blocks, then being cleared on write just in time when they are needed?

It's not that I'm worried (a secure erase should not be a problem eventually, saving/restoring a partition is something I've done a dozend times with other disks already), I'm merely curious to find out how it works.