[AlanYx]

Yes, that's the reason why a dedup vdev that has lower redundancy than your main pool will fail with "mismatched replication level" unless you use the -f (force) flag.

I'm not sure about whether an L2ARC vdev can offload the DDT, but my guess is no given the built-in logic warning against mismatched replication levels.

[archagon]

Well, the warning makes sense with respect to the dedup vdev since the DDT would actually be stored there. On the other hand, the L2ARC would simply serve as a read cache, similar to the DDT residing in RAM.