We study tracing garbage collection (GC) for a distributed shared memory (DSM) in order to provide persistence by reachability (PBR), in a large-scale distributed system. Within a general model of DSM, we specify a distributed tracing GC algorithm that scales, collects cycles, and is orthogonal to coherence. Its main features are: (i) piecewise collection of opportunistically-chosen subsets of the memory, (ii) each site collects independently of other sites, (iii) data replicas are collected independently and no coherence operation is needed for GC purposes; and (iv) asynchrony of collection with respect to applications.