A method for improving I/O performance in synchronous data replication environments is disclosed. In one embodiment, such a method includes receiving write data into a primary write cache of a primary storage system. The method synchronously mirrors the write data from the primary write cache to a secondary write cache of a secondary storage system. The method is further configured to detect when the primary write cache is full. When the primary write cache is full, the method temporarily uses primary read cache of the primary storage system to store incoming write data. This incoming write data is mirrored from the primary read cache to the secondary write cache of the secondary storage system. A corresponding system and computer program product are also disclosed herein.