Strategic LVG Fragmentation for VM

Proposed is a way to optimize virtual disk images (VDI) across a logical volume (LV) inside of a logical volume group (LVG). The trick would be to have a JBOD/LVG storage concatenation of two storage devices, one fast and small and the other slow and big. Inside of this LVG would an LV, and inside of that would be virtual disk images. The host virtualization platform would track which portions of the virtual disk image are frequently accessed and cache them to the fast disk, and it would move infrequently used portions to the slow disk. This would be transparent to the guest OS and would probably have to be implemented at the host's filesystem level (like ext4, JFS, etc.). Otherwise, in the host OS, the disk image would still be accessible as a single file for image backups and the like. This would be more granular than selecting which files to cache (like operating systems probably do); a particular PORTION of a database tablespace could be cached at a low level instead of either trying to keep the whole tablespace in RAM or trying to cache rows at the DB level.