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Single system image

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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.

Features of SSI clustering systems[edit]

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

Process migration[edit]

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.

Process checkpointing[edit]

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.

Single process space[edit]

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.

Single root[edit]

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.

Single I/O space[edit]

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).

Single IPC space[edit]

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.

Cluster IP address[edit]

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 2018 NonStop OS
Inferno No No No Yes Yes Yes Un­known Open March 4, 2015 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
Open-Sharedroot[t 9] 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 9, 2015 Native
Sprite Yes Un­known No Yes Yes No Un­known Open 1992 Native
TidalScale 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 January 25, 2024 OpenVMS
z/VM Yes No Yes No No Yes Un­known Closed September 16, 2022 Native
UnixWare NonStop Clusters[t 10] Yes No Yes Yes Yes Yes Yes Closed June 2000 UnixWare
  1. ^ Many of the Linux based SSI clusters can use the Linux Virtual Server to implement a single cluster IP address
  2. ^ Green means software is actively developed
  3. ^ Amoeba development is carried forward by Dr. Stefan Bosse at BSS Lab Archived 2009-02-03 at the Wayback Machine
  4. ^ Guardian90 TR90.8 Based on R&D by Tandem Computers c/o Andrea Borr at [1]
  5. ^ LinuxPMI is a successor to openMosix
  6. ^ LOCUS was used to create IBM AIX TCF
  7. ^ LOCUS used named pipes for IPC
  8. ^ openMosix was a fork of MOSIX
  9. ^ Open-Sharedroot is a shared root Cluster from ATIX
  10. ^ UnixWare NonStop Clusters was a base for OpenSSI

See also[edit]


  1. ^ for example it may be necessary to move long running processes off a node that is to be closed down for maintenance
  2. ^ Checkpointing is particularly useful in clusters used for high-performance computing, avoiding lost work in case of a cluster or node restart.
  3. ^ "leaving a cluster" is often a euphemism for crashing


  1. ^ Pfister, Gregory F. (1998), In search of clusters, Upper Saddle River, NJ: Prentice Hall PTR, ISBN 978-0-13-899709-0, OCLC 38300954
  2. ^ Buyya, Rajkumar; Cortes, Toni; Jin, Hai (2001), "Single System Image" (PDF), International Journal of High Performance Computing Applications, 15 (2): 124, doi:10.1177/109434200101500205, S2CID 38921084
  3. ^ Healy, Philip; Lynn, Theo; Barrett, Enda; Morrison, John P. (2016), "Single system image: A survey" (PDF), Journal of Parallel and Distributed Computing, 90–91: 35–51, doi:10.1016/j.jpdc.2016.01.004, hdl:10468/4932
  4. ^ Coulouris, George F; Dollimore, Jean; Kindberg, Tim (2005), Distributed systems: concepts and design, Addison Wesley, p. 223, ISBN 978-0-321-26354-4
  5. ^ Bolosky, William J.; Draves, Richard P.; Fitzgerald, Robert P.; Fraser, Christopher W.; Jones, Michael B.; Knoblock, Todd B.; Rashid, Rick (1997-05-05), "Operating System Directions for the Next Millennium", 6th Workshop on Hot Topics in Operating Systems (HotOS-VI), Cape Cod, MA, pp. 106–110, CiteSeerX, doi:10.1109/HOTOS.1997.595191, ISBN 978-0-8186-7834-9, S2CID 15380352{{citation}}: CS1 maint: location missing publisher (link)
  6. ^ Prabhu, C.S.R. (2009), Grid And Cluster Computing, Phi Learning, p. 256, ISBN 978-81-203-3428-1
  7. ^ Smith, Jonathan M. (1988), "A survey of process migration mechanisms" (PDF), ACM SIGOPS Operating Systems Review, 22 (3): 28–40, CiteSeerX, doi:10.1145/47671.47673, S2CID 6611633
  8. ^ "AIX PS/2 OS".
  9. ^ "Open-Sharedroot GitHub repository". GitHub.
  10. ^ Pike, Rob; Presotto, Dave; Thompson, Ken; Trickey, Howard (1990), "Plan 9 from Bell Labs", In Proceedings of the Summer 1990 UKUUG Conference, p. 8, Process migration is also deliberately absent from Plan 9. {{citation}}: Missing or empty |title= (help)