[Filligree]

I remember this code!

If we're talking about my desktop, its current configuration is 3x 2TB NVMe (configured as zfs cache) plus 2x 12TB HDDs (mirrored). I've set sync=disabled, with transaction groups committing every 10 minutes — this is fine for my use case — so the HDDs spend most of their time spun down.

I only actually have 4TB of data on the system. It keeps growing, but the working set is probably much less than that.

Which means, it's 100% cached. A single read sent to the HDDs would have a latency of multiple seconds; absolutely catastrophic for a desktop workload. In this case _always_ using the cache _is_ the right answer, but I've been trying to think of an algorithm that would be able to do so without hardcoding it.