1. A method of dynamic allocation of non-volatile memory in a cache storage subsystem, comprising: constraining a redundant array of independent disks (RAID) 6 array in an unavailable state, the RAID 6 array having been newly created in the subsystem; destaging data from a write cache to a storage medium apart of the RAID 6 array to free memory space in the write cache, dynamically reducing the size of the write cache to accommodate persistent data; reserving the freed memory space for allocation to store the persistent data, the persistent data including Atomic Parity Update APU data; allocating the freed memory space to an adapter in the subsystem as a target for the persistent data in the event of a power loss; and once the freed memory space is determined to have been allocated, changing the RAID 6 array to an available state.