A system, apparatus and method for managing input/output requests in a multi-processor system is disclosed. An IO coherence unit includes an IO request handler, a variable size transaction table, and an IO response handler. The size of the transaction table varies according to the number of pending IO requests. The IO request handler stores information about pending IO requests in the transaction table to establish an order among related requests and to permit out-of-order handling of unrelated requests. The IO response handler tracks responses to the IO requests and updates the information in the transaction table. The IO coherence unit returns responses to requesting devices in compliance with device ordering requirements.