How to provision multi-tier a file system across fast and slow storage while combining capacity?

I'm hunting for a way to utilise a slow 500GB magnetic HDD alongside a fast 500GB SSD. I'd like to end up with a reasonable fraction of the two combined [hopefully > 800GB] in terms of capacity, but **deciding which files should live on which disk is going to be really tricky and near impossible to cut across directory boundaries**. It occurs to be that something like this has already been thought of in the form of multi-tired storage. But so far the only mechanisms I've found use the fast volume as a cache over the slow volume. That's NOT useful to me since I'd just end up with 500GB of SSD cache over 500GB HDD slow drive... I'd be no better off than simply binning the slow HDD and using only the SSD. Hypothetically a system could exist that spanned across two volumes and dynamically copied from one to the other. This would be similar to a cache but without any requirement to keep one volume wholly consistent, and with the ability to provision more capacity than either single volume. So far all my searches have drawn blanks. I've found multiple references to LVM and XFS supporting caching. But nothing obviously has a path to achieving higher capacity than either single device. So while it's hypothetically possible, I don't see a way to achieve it. --------- *Note that the tricky feature of this question is achieving a solution with only one end file system. Manually choosing which files go where by directory will not be possible for me.*