You said "Don't use QCOW2, just don't, it's slow and you're just adding extra layers where you don't need to.". I was agreeing with you when I said that it also

You said "Don't use QCOW2, just don't, it's slow and you're just adding extra layers where you don't need to.". I was agreeing with you when I said that it also has that effect on ZFS. What is my "assumption about QCOW2 and BTRFS performance"? As for newer kernels, code churn tends to break things that previously worked, which upsets customers who want bugs to decrease as a function of time, rather than go up. Vendors like Redhat will not update distributions like RHEL to newer kernels because of that and they will not support kernels that they do not ship. That is why people run older kernels.