> A fast SSD can be set as the target for foreground writes, but that data will be transparently copied in the background to a "background" target, i.e. a large/slow disk.

This is very similar in concept to (or an evolution of?) ZFS's ZIL:

* https://www.servethehome.com/what-is-the-zfs-zil-slog-and-wh...

* https://www.truenas.com/docs/references/zilandslog/

* https://www.45drives.com/community/articles/zfs-caching/

When this feature was first introduced to ZFS in the Solaris 10 days there was an interesting demo from a person at Sun that I ran across: he was based in a Sun office on the US East Coast where he did stuff, but had access to Sun lab equipment across the US. He mounted iSCSI drives that were based in (IIRC) Colorado as a ZFS poool, and was using them for Postgres stuff: the performance was unsurprisingly not good. He then add a local ZIL to the ZFS pool and got I/O that was not too far off from some local (near-LAN) disks he was using for another pool.