Single system image

In distributed computing, a single system image (SSI) cluster is a cluster of machines that appears to be one single system.^[1][2][3] The concept is often considered synonymous with that of a distributed operating system,^[4][5] 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.^[6] 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.^[7] Processes may start on one node and be moved to another node, possibly for resource balancing or administrative reasons.^{[note 1]} 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.^{[note 2]} 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 NFS 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.

Some SSI systems provide a way of "breaking the illusion", having some node-specific files even in a single root. HP TruCluster provides a "context dependent symbolic link" (CDSL) which points to different files depending on the node that accesses it. HP VMScluster provides a search list logical name with node specific files occluding cluster shared files where necessary. This capability may be necessary to deal with heterogeneous clusters, where not all nodes have the same configuration. In more complex configurations such as multiple nodes of multiple architectures over multiple sites, several local disks may combine to form the logical single root.

Some SSI systems allow all nodes to access the I/O devices (e.g. tapes, disks, serial lines and so on) of other nodes. There may be some restrictions on the kinds of accesses allowed (For example, OpenSSI can't mount disk devices from one node on another node).

Some SSI systems allow processes on different nodes to communicate using inter-process communications mechanisms as if they were running on the same machine. On some SSI systems this can even include shared memory (can be emulated in software with distributed shared memory).

In most cases inter-node IPC will be slower than IPC on the same machine, possibly drastically slower for shared memory. Some SSI clusters include special hardware to reduce this slowdown.

Some SSI systems provide a "cluster IP address", a single address visible from outside the cluster that can be used to contact the cluster as if it were one machine. This can be used for load balancing inbound calls to the cluster, directing them to lightly loaded nodes, or for redundancy, moving the cluster address from one machine to another as nodes join or leave the cluster.^{[note 3]}

Examples here vary from commercial platforms with scaling capabilities, to packages/frameworks for creating distributed systems, as well as those that actually implement a single system image.

SSI Properties of different clustering systems

Name	Process migration	Process checkpoint	Single process space	Single root	Single I/O space	Single IPC space	Cluster IP address[t 1]	Source Model	Latest release date[t 2]	Supported OS
Amoeba[t 3]	Yes	Yes	Yes	Yes	Unknown	Yes	Unknown	Open	July 30, 1996	Native
AIX TCF	Unknown	Unknown	Unknown	Yes	Unknown	Unknown	Unknown	Closed	March 30, 1990[8]	AIX PS/2 1.2
NonStop Guardian[t 4]	Yes	Yes	Yes	Yes	Yes	Yes	Yes	Closed	September 2025	NonStop OS
Inferno	No	No	No	Yes	Yes	Yes	Unknown	Open	August 13, 2025	Native, Windows, Irix, Linux, OS X, FreeBSD, Solaris, Plan 9
Kerrighed	Yes	Yes	Yes	Yes	Unknown	Yes	Unknown	Open	June 14, 2010	Linux 2.6.30
LinuxPMI[t 5]	Yes	Yes	No	Yes	No	No	Unknown	Open	June 18, 2006	Linux 2.6.17
LOCUS[t 6]	Yes	Unknown	Yes	Yes	Yes	Yes[t 7]	Unknown	Closed	1988	Native
MOSIX	Yes	Yes	No	Yes	No	No	Unknown	Closed	October 24, 2017	Linux
openMosix[t 8]	Yes	Yes	No	Yes	No	No	Unknown	Open	December 10, 2004	Linux 2.4.26
OpenPMIx[t 9]	Yes	Yes	No	Yes	No	No	Unknown	Open	September 26, 2025	Linux
Open-Sharedroot[t 10]	No	No	No	Yes	No	No	Yes	Open	September 1, 2011[9]	Linux
OpenSSI	Yes	No	Yes	Yes	Yes	Yes	Yes	Open	February 18, 2010	Linux 2.6.10 (Debian, Fedora)
Plan 9	No[10]	No	No	Yes	Yes	Yes	Yes	Open	January 10, 2015	Native
Sprite	Yes	Unknown	No	Yes	Yes	No	Unknown	Open	1992	Native
TidalScale (now HPE)	Yes	No	Yes	Yes	Yes	Yes	Yes	Closed	August 17, 2020	Linux, FreeBSD
TruCluster	No	Unknown	No	Yes	No	No	Yes	Closed	October 1, 2010	Tru64
VMScluster	No	No	Yes	Yes	Yes	Yes	Yes	Closed	November 20, 2024	OpenVMS
z/VM	Yes	No	Yes	No	No	Yes	Unknown	Closed	September 30, 2024	Native
UnixWare NonStop Clusters[t 11]	Yes	No	Yes	Yes	Yes	Yes	Yes	Closed	June 2000	UnixWare

• Diskless shared-root cluster
• Distributed lock manager
• Distributed cache
• Parallel Virtual Machine - multiple system image alternative
• Message Passing Interface - multiple system image alternative