In distributed computing, a single system image (SSI) cluster is a cluster of machines that appears to be one single system. The concept is often considered synonymous with that of a distributed operating system, but a single image may be presented for more limited purposes, just job scheduling for instance, which may be achieved by means of an additional layer of software over conventional operating system images running on each node. The interest in SSI clusters is based on the perception that they may be simpler to use and administer than more specialized clusters.

openMosix is a free cluster management system that provided single-system image (SSI) capabilities, e.g. automatic work distribution among nodes. It allowed program processes (not threads) to migrate to machines in the node's network that would be able to run that process faster (process migration). It was particularly useful for running parallel applications having low to moderate input/output (I/O). It was released as a Linux kernel patch, but was also available on specialized Live CDs. openMosix development has been halted by its developers, but the LinuxPMI project is continuing development of the former openMosix code.