Software define filesystems for filesserver?

Let's say we have a fileserver with a bunch of PCIe slots with 2x 10GE 2s nics, link aggregate/bonding to get 40Gbps.

For storage, I have a bunch of hdd drives, and 4x1tb-PCI-e ssd.

I mirrored the pcie-ssd into two set. I want my filesystem to behave like the following:

All the writes will be performed into one of the mirror pcie-ssd set. When the write reach a certain limit (say 800GB) for example, the next files will be written to the second set of the mirrored pcie-ssd. While there is data on the unused mirrored-pcie-ssd, it will start to move it's data to the slow hdd. Using ram can also be an option.

I understand the above example might not be ideal for fileserver because what about read? however, I just want to know whether something like that is possible. btrfs/zfs or anything is fine.