Single system image

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.

Different SSI systems may provide a more or less complete illusion of a single system.

Different SSI systems may, depending on their intended usage, provide some subset of these features.

Many SSI systems provide process migration. Processes may start on one node and be moved to another node, possibly for resource balancing or administrative reasons. As processes are moved from one node to another, other associated resources (for example IPC resources) may be moved with them.

Some SSI systems allow checkpointing of running processes, allowing their current state to be saved and reloaded at a later date. Checkpointing can be seen as related to migration, as migrating a process from one node to another can be implemented by first checkpointing the process, then restarting it on another node. Alternatively checkpointing can be considered as migration to disk.

Some SSI systems provide the illusion that all processes are running on the same machine - the process management tools (e.g. "ps", "kill" on Unix like systems) operate on all processes in the cluster.

Most SSI systems provide a single view of the file system. This may be achieved by a simple server, shared disk devices or even file replication.

The advantage of a single root view is that processes may be run on any available node and access needed files with no special precautions. If the cluster implements process migration a single root view enables direct accesses to the files from the node where the process is currently running.

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