[riobard]

If I partition the entire drive, eventually all blocks will be used, depending on how the filesystem allocates, right? So to guarantee some free space it's better to over-provision by under-partitioning. Now how do I make sure that on a used drive?

[rdc12]

You could use some sort of disk quota system, to make the filesystem artificially smaller than it actually is (trim after applying this change). Or simply insure that you don't exceed 80% - 90% used space.

It it is also worth noting that many SSD's are over-provisioned by the manufacture anyway, in those drives manual over-provisioning might achieve very little anyway.

[ars]

That's what the trim command does.

It runs periodically and lets the SSD know about unused areas.

So as long as you don't fill up the drive and let trim do its thing the unused areas effectively do the same thing as over provisioning.