System and methods for managing I/O write requests of host systems to physical storage. A storage subsystem includes a plurality of storage devices where each storage device is configured to provide data storage. At least a pair of redundant controllers is connected to the plurality of storage devices for executing the I/O write requests from the host systems. A received I/O write request is initially saved in a controller memory of one of the controllers and mirrored in controller memory of the other controller. In one embodiment, the I/O write request is transferred to a flash memory device for subsequent transfer to the storage devices. Once transferred to the flash memory device, the I/O write request may be flushed from the controller memories. The I/O write request may then be transferred to the storage devices from the flash memory device as a background operation.