BitTorrent plays a very important role in the current Internet content distribution. The enormous impact of public and private trackers should not be overlooked. Public trackers are suffering from free-riding problem, but private trackers are becoming more and more popular and they run very well in terms of an effective Share Ratio Enforcement (SRE) which is an auxiliary incentive mechanism. In this paper, we have crawled and traced 15 trackers with 3.5 million torrents for over 6 months. We first provide taxonomy of private trackers, and then present in breadth and depth measurement from the user viscosity, torrents evolution, user behaviors, content distribution and other metrics. Some features are apparently different from public trackers. Furthermore, we analyze SRE mechanism and point/credit system, and use game theory to study the effectiveness of SRE. There exists "uploading starvation" phenomenon in private trackers. We model SRE mechanism and preliminary propose an improved SRE mechanism to further incent users and enhance the performance of private trackers.