An apparatus, method and computer program in a distributed cluster storage network comprises storage control nodes to write data to storage on request from a host; a forwarding layer at a first node to forward data to a second node; a buffer controller at each node to allocate buffers for data to be written; and a communication link between the buffer controller and the forwarding layer at each node to communicate a constrained or unconstrained status indicator of the buffer resource to the forwarding layer. A mode selector selects a constrained mode of operation requiring allocation of buffer resource at the second node and communication of the allocation before the first node can allocate buffers and forward data, or an unconstrained mode of operation granting use of a predetermined resource credit provided by the second to the first node and permitting forwarding of a write request with data.