Understanding IO patterns and performance of CMS Data Analysis across T2s worldwide

We discuss the IO patterns and performance of CMS Data Analysis based on measurements on "the bench" as well as operational observations during the initial LHC data taking period. It was observed the default I/O settings for CMS data analysis resulted in poor CPU efficiency and high I/O load at many T2 sites. This effort developed tools to measure CMSSW I/O performance for grid-based analysis. Our work enabled CMSSW developers to understand how new I/O strategies work across the full range of deployed T2 storage solutions. We discuss our results, based upon aggregate I/O statistics collected worldwide.