Cluster-based storage systems connected with TCP/IP networks are expected to achieve a high throughput by striping files across multiple storage servers. However, for the storage system interconnected with the TCP/IP network, several critical issues, like Incast effect and data access interference, invalidate the assumption that higher access parallelism always results in increased I/O throughput. To address this issue, we propose a new file striping strategy, named as storage server grouping (SSG), which changes file striping pattern across the storage servers based on the analysis of file popularity and impact of the number of storage servers on the clients' perceived performance (I/O speedup) to reduce the interference of data accesses to popular files and avoid dramatic reduction of system throughput caused by the Incast effect. Our experimental evaluation shows that SSG can improve I/O throughput by 22.1% on average.