Software DSM provides good programmability for cluster computing, but its performance and limited shared memory space for large applications hinder its popularity. This paper introduces LOTS, a C++ runtime library supporting a large shared object space. With its dynamic memory mapping mechanism, LOTS can map more objects, lazily from the local disk to the virtual memory during access, leaving only a trace of control information for each object in the local process space. To our knowledge, LOTS is the first pure runtime software DSM supporting a shared object space larger than the local process space. Our testing shows that LOTS can utilize all the free hard disk space available to support hundreds of gigabytes of shared objects with a small overhead. The scope consistency memory model and a mixed coherence protocol allow LOTS to achieve better scalability with respect to problem size and cluster size.