We need an option exposed in the UI for special (metadata vdev) in dataset create/update:
special_small_blocks=size This value represents the threshold block size for including small file blocks into the special allocation class. Blocks smaller than or equal to this value will be assigned to the special allocation class while greater blocks will be assigned to the regular class. Valid values are zero or a power of two from 512B up to 1M. The default size is 0 which means no small file blocks will be allocated in the special class. Before setting this property, a special class vdev must be added to the pool. See zpool(8) for more details on the special allocation class.
in the API its called "special_small_block_size"
We should call it "Metadata (Special) Small Block Size"