The mq-deadline part is what confused me. That is a competing option for the Linux IO elevator that runs under ZFS. Anyway, I understand now and thanks for the data point. I added a list of questions to GitHub that could help narrow things down if you take time to answer them. I will be watching the GitHub thread regularly, so I will see it if you post the answers after you return from your travels.

I was confused, actually, not you. The output was:

cat /sys/dev/block/8:176/queue/scheduler

[mq-deadline] none

However, this output does not mean what I thought it did - it means that mq-deadline is in use.

If I do

echo "none" | sudo tee /sys/dev/block/8:176/queue/scheduler

This changes to

cat /sys/dev/block/8:176/queue/scheduler

[none] mq-deadline