ZFS’ deduplication does not require the deduplication table be held in RAM. Hoewever, the performance is IOPS bound. The only way to improve on that is to keep it in RAM or have a very fast L2ARC. Many modifications also require quite a large number of writes, which also slows things down.

My point is that the system will not stop working if the DDT is not in cache. It would still run, but it would not be fast.