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The popularity of Web services demands data storage over the network. iSCSI is a newly emerging protocol with the goal of deploying storage over the mature TCP/IP in an economic and convenient way. In a typical Web service using iSCSI storage systems, I/O requests issued by Web users can be satisfied by different parties, such as client cache, proxy cache, Web server cache, iSCSI target cache, and disks, along the I/O path from Web clients to iSCSI storage. Understanding the characteristics of I/O traffic under such environment is increasingly important as the networked storage prevails. This paper profiles I/O for Web services using iSCSI storage systems and identifies how these I/O requests are distributed and satisfied. Both simulation and benchmarking results show that the I/O profiling is helpful to identify I/O performance bottleneck of Web services and helps to improve iSCSI performance.