> How is L2ARC not "true hybrid"? It doesn't persist across import/export or reboot. It is demand-filled. Not all data in the main vdevs are eligible for l2a

> How is L2ARC not "true hybrid"? It doesn't persist across import/export or reboot. It is demand-filled. Not all data in the main vdevs are eligible for l2arc. There is memory overhead for l2arc buffers. There is CPU overhead in processing l2arc headers. Once a buffer is in l2arc it stays in l2arc until the underlying data is overwritten or destroyed, or until the l2arc has filled up and the buffer is replaced with fresher data. A true hybrid in the zfs context would let one pin a dataset or zvol onto a particular vdev, or pin only the (zfs) metadata (or a subset thereof) of a dataset, zvol or pool to a particular vdev. Openzfs will eventually get both persistence and this form of true hybrid. Unfortunately automatic migration by zfs of hot data to low-latency vdevs and cool data from low-latency vdevs is not really possible without solving the infamous block-pointer-rewrite problem.