Thank for your reply, if the whole pool is fixed then the use case for it would be for enterprise user that can design a pool with many disks up front, instead of adding them on demand. Thus not so useful for normal userDRAID is very new. I don't know whether the layout will be capable of adjusting after a disk is added. I vaguely remember that there was a hint (I think in Isaak Huang's presentation) saying that the size of the pool is fixed once created. But that may very well be a temporary state during development.
I know that it is technically possible to change the capacity of a declustered RAID system while running, but it is quite complex to do.