Excellent! I was just investigating btrfs[0] and realized that it doesn't offer anything like ZFS's "2nd level storage" (L2ARC) which is doing precisely this: keep what is nowadays referred to as hot data on dedicated (read: solid state) storage; as much as possible.

In the course of this little recherché I hit upon bcache and dm-cache but found (Debian) user-land to be entirely lacking (`modprobe dm-cache` doesn't raise an eyebrow, though).

Having dm-cache support in LVM -- one of those projects that figure centrally in my personal sysadmin lore -- would be just heavenly.

[0] Balanced Tree FS. Similar in outlook and approach to SUN-now-Oracle's ZFS and -- oh, irony of ironies -- also under the Oracle umbrela.