HDD spin up/down and caching (ZFS)

I’ve been focusing on getting power utilization down and one of the keys to this is disk spin down.

I have set all the max power options and these work, but the problem is, if I even think about the HDD , it spins up.

I assume that anything checking NFS / SMB is still alive might be the culprit, along with bring lists of files / folders to the foreground.

I recall from one of Toms video’s he talks about using SSD’s for caching some information with ZFS. I wondered if using an SSD for caching indexes might stop the disk being spun up unless there is an actual read or write operation. That said I could easily assign more RAM.

However reading about ARC, it talks about caching data rather than indexes.

Are there any strategies that would cache the disk index in RAM / SSD and prevent disk spin up unless something actually needs reading or writing? (I dont care about the lag).

Any other options I should consider?

Not something I have any experience with or the time to test. I shut down systems that are not in use to save power.