Abstract
Tier-2 to Tier-2 data transfers have been identified as a necessary extension of the CMS computing model. The Debugging Data Transfers (DDT) Task Force in CMS was charged with commissioning Tier-2 to Tier-2 PhEDEx transfer links beginning in late 2009, originally to serve the needs of physics analysis groups for the transfer of their results between the storage elements of the Tier-2 sites associated with the groups. PhEDEx is the data transfer middleware of the CMS experiment. For analysis jobs using CRAB, the CMS Remote Analysis Builder, the challenges of remote stage out of job output at the end of the analysis jobs led to the introduction of a local fallback stage out, and will eventually require the asynchronous transfer of user data over essentially all of the Tier-2 to Tier-2 network using the same PhEDEx infrastructure. In addition, direct file sharing of physics and Monte Carlo simulated data between Tier-2 sites can relieve the operational load of the Tier-1 sites in the original CMS Computing Model, and already represents an important component of CMS PhEDEx data transfer volume. The experience, challenges and methods used to debug and commission the thousands of data transfers links between CMS Tier-2 sites world-wide are explained and summarized. The resulting operational experience with Tier-2 to Tier-2 transfers is also presented.