Costa: Designing a Userspace Disk I/O Scheduler for Modern Datastores: the Scylla example (Part 1)

There are even circumstances where it would actually be reasonably sane to use ZFS (the volume layer) as your volume management layer, and ext4 as your filesystem layer, aside from the obvious like exporting the volume to a VM or an iSCSI client that just does its own thing.

I've seriously considered it for storing my email backups where more than 50% of the space usage is waste due to internal fragmentation, where ZFS (the filesystem layer) suffers fairly badly compared to ext4. Ultimately, losing low double digit gigabytes isn't a pressing concern these days so I've not bothered, but I can well imagine there would be circumstances where ext4 or some other filesystem would be a sensible choice.