1. A method of destaging a computer readable write cache in a storage system having an number of arrays of storage disks, such an array having a rank of data ordered in a sequence of write groups, wherein the write cache is shared by the disk arrays, the method comprising the steps of: a) storing ones of the write groups in the write cache temporarily responsive to the ones of the write groups being written to their respective arrays; b) assigning the write groups stored in the write cache to a global queue, wherein the write groups in the global queue are ordered by ages of the write groups; c) selecting a quantity of write groups for attempted destaging to the arrays responsive to a predetermined high threshold for the global queue and to sizes and the ages of the write groups in the global queue; d) allocating the selected quantity among the arrays responsive to quantities of certain ones of the write groups in the global queue, so that the arrays have respective allocation quantities of write groups for destaging; and e) destaging ones of the write groups in the write cache to the arrays, wherein write groups are destaged to such an array responsive to i) the selected allocation quantity for the array and ii) the sequences of the write groups in the array.