Currently, AFAIK, the dedup table (DDT) has to reside entirely in RAM. The L2ARC is a disk-based cache of filesystem blocks (a pretty clever design, worth read

Currently, AFAIK, the dedup table (DDT) has to reside entirely in RAM. The L2ARC is a disk-based cache of filesystem blocks (a pretty clever design, worth reading up on), but a very different thing from the DDT. You could definitely put the L2ARC on Optane, but Optane is probably better suited to the ZIL (ZFS Intent Log, basically the journal so writes can be quickly acked) if you aren't using literal NVRAM. Breaking the DDT out so it could exist on a Optane isn't currently available (again, AFAIK, but I recall hearing about some changes in that department, I don't recall specifics and may be misremembering). I've basically never had a machine big enough to comfortably use dedup except on trivial loads. My backup boxes, where I'd really like to use it, have all fallen over when enabling dedup because of the RAM requirements.