Abstract Volunteer computing is a type of large-scale heterogeneous distributed system where resources necessary to run the system are donated by volunteers. A draw back of volunteer computing is the unreliability of the donated resources, so redundancy is required to guarantee the fulfillment of tasks or the availability of data In this work we consider the problem of designing a directory service policy for a Distributed Volunteer Computing Micro-Blogging Service (DVCMBS). In such services, nodes donate storage space (repositories), which is managed by a centralized directory service that decides which nodes will store replicas of blogs, ensuring their online availability when bloggers are offline. Since nodes are under no obligation to remain online, the task of a DVCMBS directory service is to allocate blog replicas to online repositories, such that the rate of availability of all blogs is maximized. At the same time, since donated storage resources are limited and directory service operations use processing resources, minimizing the number of blog replicas generated ( i . e . , the inefficiency of the directory service) is critical. We present a simulation model of a DVCMBS, which uses a probabilistic sort and select approach to select host repositories for replicas and blogs to replicate. Exhaustive computational experiments analyze the trade-off between blog replica availability and efficiency, and identify the efficient directory service policies, with respect to availability and efficiency maximization.