Editing Single system image

{{Short description|Cluster dedicated operating system}}
In [[distributed computing]], a '''single system image''' ('''SSI''') cluster is a [[cluster (computing)|cluster]] of machines that appears to be one single system.<ref name=Pfister1998>{{Citation
 | last = Pfister
 | first = Gregory F.
 | year = 1998
 | title = In search of clusters
 | isbn = 978-0-13-899709-0
 | publisher = Prentice Hall PTR
 | location = Upper Saddle River, NJ
 | oclc = 38300954
 | url-access = registration
 | url = https://archive.org/details/insearchofcluste00pfis
 }}</ref><ref name=Buyya2001>{{citation
 |author1=Buyya, Rajkumar |author2=Cortes, Toni |author3=Jin, Hai | year = 2001
 | title = Single System Image
 | journal = International Journal of High Performance Computing Applications
 | volume = 15
 | issue = 2
 | pages = 124
 | doi = 10.1177/109434200101500205
 |s2cid=38921084 | url = http://www.buyya.com/papers/SSI-CCWhitePaper.pdf
}}</ref><ref name=Healy2016>{{citation
 |author1=Healy, Philip |author2=Lynn, Theo |author3=Barrett, Enda |author4=Morrison, John P. | year = 2016
 | title = Single system image: A survey
 | journal = Journal of Parallel and Distributed Computing
 | volume = 90-91
 | pages = 35–51
 | doi = 10.1016/j.jpdc.2016.01.004
 | url = https://cora.ucc.ie/bitstream/handle/10468/4932/PH_Single_SV2016.pdf
|hdl=10468/4932 }}</ref> The concept is often considered synonymous with that of a [[distributed operating system]],<ref>
{{Citation
 | title = Distributed systems: concepts and design
 | year = 2005
 |author1=Coulouris, George F |author2=Dollimore, Jean |author3=Kindberg, Tim | isbn = 978-0-321-26354-4
 | publisher = Addison Wesley
 | page = 223
 | url = https://books.google.com/books?id=d63sQPvBezgC&dq=%22distributed+operating+system%22&pg=PA223
}}</ref><ref>{{Citation
 |author1=Bolosky, William J. |author2=Draves, Richard P. |author3=Fitzgerald, Robert P. |author4=Fraser, Christopher W. |author5=Jones, Michael B. |author6=Knoblock, Todd B. |author7=Rashid, Rick | contribution = Operating System Directions for the Next Millennium | title = 6th Workshop on Hot Topics in Operating Systems (HotOS-VI) | place = Cape Cod, MA | pages = 106–110 | date = 1997-05-05| doi = 10.1109/HOTOS.1997.595191  | citeseerx = 10.1.1.50.9538 
|isbn=978-0-8186-7834-9 |s2cid=15380352 }}</ref> 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|operating system image]]s running on each [[Node (networking)|node]].<ref>{{Citation
 | title=Grid And Cluster Computing
 | author= Prabhu, C.S.R.
 | isbn=978-81-203-3428-1
 | publisher=Phi Learning
 | year=2009
 | pages=256
 | url=https://books.google.com/books?id=EIVdVtGHv-0C&dq=%22distributed+operating+system%22+%22single+system+image%22&pg=PA177
}}</ref> 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==

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

===Process migration===
{{main|Process migration}}
Many SSI systems provide [[process migration]].<ref name=Smith1988>{{citation
 | last = Smith | first =  Jonathan M.
 | year = 1988
 | title = A survey of process migration mechanisms
 | journal = ACM SIGOPS Operating Systems Review
 | volume = 22
 | issue =  3
 | pages = 28–40
 | doi = 10.1145/47671.47673
 | url = http://www.cis.upenn.edu/~jms/svy-pm.pdf
| citeseerx =  10.1.1.127.8095
 | s2cid = 6611633
 }}</ref>
Processes may start on one [[Node (networking)|node]] and be moved to another node, possibly for [[Load balancing (computing)|resource balancing]] or administrative reasons.<ref group="note">for example it may be necessary to move long running processes off a node that is to be closed down for maintenance</ref>  As processes are moved from one node to another, other associated resources (for example [[Inter-process communication|IPC]] resources) may be moved with them.

===Process checkpointing===
Some SSI systems allow [[Application checkpointing|checkpointing]] of running processes, allowing their current state to be saved and reloaded at a later date.<ref group="note">Checkpointing is particularly useful in clusters used for [[high-performance computing]], avoiding lost work in case of a cluster or node restart.</ref>
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===
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===
Most SSI systems provide a single view of the file system. This may be achieved by a simple [[Network File System (protocol)|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. [[Hewlett-Packard|HP]] [[TruCluster]] provides a "context dependent symbolic link" (CDSL) which points to different files depending on the node that accesses it.  [[Hewlett-Packard|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===
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===
Some SSI systems allow processes on different nodes to communicate using [[inter-process communication]]s mechanisms as if they were running on the same machine.  On some SSI systems this can even include [[Shared memory (interprocess communication)|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===
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.<ref group=note>"leaving a cluster" is often a euphemism for crashing</ref>

==Examples==

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.

{|class="wikitable sortable"
 |+SSI Properties of different clustering systems
 |-
 !Name
 ![[#Process migration|Process migration]]
 ![[#Process checkpointing|Process checkpoint]]
 ![[#Single process space|Single process space]]
 ![[#Single root|Single root]]
 ![[#Single I/O space|Single I/O space]]
 ![[#Single IPC space|Single IPC space]]
 ![[#Cluster IP address|Cluster IP address]]<ref group="t">Many of the [[Linux]] based SSI clusters can use the [[Linux Virtual Server]] to implement a single cluster IP address</ref> 
 !Source Model
 !Latest release date<ref group="t">Green means software is actively developed</ref>
 !Supported OS 
 |-
 |[[Amoeba distributed operating system|Amoeba]]<ref group="t">[[Amoeba distributed operating system|Amoeba]] development is carried forward by Dr. Stefan Bosse at [http://www.bsslab.de/english/index.html BSS Lab] {{webarchive|url=https://web.archive.org/web/20090203124419/http://bsslab.de/english/index.html |date=2009-02-03 }}</ref>
 | {{Yes}} <!-- Process migration -->
 | {{Yes}} <!-- Process checkpointing -->
 | {{Yes}} <!-- Single process space -->
 | {{Yes}} <!-- Single root -->
 | {{Unk}} <!-- Single I/O space -->
 | {{Yes}} <!-- Single IPC space -->
 | {{Unk}} <!-- Cluster IP address -->
 | {{Yes|Open}} <!-- Source Model -->
 | {{No|{{dts|1996|07|30}}}} <!-- Latest release date -->
 | Native <!-- Supported OS -->
 |-
 |[[AIX]] [[Transparent Computing Facility|TCF]]
 | {{Unk}} <!-- Process migration -->
 | {{Unk}} <!-- Process checkpointing -->
 | {{Unk}} <!-- Single process space -->
 | {{Yes}} <!-- Single root -->
 | {{Unk}} <!-- Single I/O space -->
 | {{Unk}} <!-- Single IPC space -->
 | {{Unk}} <!-- Cluster IP address -->
 | {{No|Closed}} <!-- Source Model -->
 | {{No|{{dts|1990|03|30}}}}<ref>{{cite web|url=http://www-01.ibm.com/common/ssi/cgi-bin/ssialias?infotype=OC&subtype=NA&htmlfid=897/ENUS5706-297&appname=totalstorage |title=AIX PS/2 OS}}</ref> <!-- Latest release date -->
 | [[AIX]] PS/2 1.2 <!-- Supported OS -->
 |-
 |[[NonStop (server computers)|NonStop Guardian]]<ref group="t">[[Guardian90 TR90.8]] Based on R&D by Tandem Computers c/o Andrea Borr at [https://www.hpl.hp.com/techreports/tandem/TR-90.8.pdf] </ref>
 | {{Yes}}<!-- Process migration -->
 | {{Yes}} <!-- Process checkpointing -->
 | {{Yes}} <!-- Single process space -->
 | {{Yes}} <!-- Single root -->
 | {{Yes}} <!-- Single I/O space -->
 | {{Yes}} <!-- Single IPC space -->
 | {{Yes}} <!-- Cluster IP address -->
 | {{No|Closed}} <!-- Source Model -->
 | {{Yes|{{dts|2018||}}}} <!-- Latest release date -->
 | [[NonStop OS]] <!-- Supported OS -->
 |-

 |[[Inferno (operating system)|Inferno]]
 | {{No}}<!-- Process migration -->
 | {{No}} <!-- Process checkpointing -->
 | {{No}} <!-- Single process space -->
 | {{Yes}} <!-- Single root -->
 | {{Yes}} <!-- Single I/O space -->
 | {{Yes}} <!-- Single IPC space -->
 | {{Unk}} <!-- Cluster IP address -->
 | {{Yes|Open}} <!-- Source Model -->
 | {{Yes|{{dts|2015|03|04}}}} <!-- Latest release date -->
 | Native, [[Windows]], [[Irix]], [[Linux]], [[OS X]], [[FreeBSD]], [[Solaris (operating system)|Solaris]], [[Plan 9 from Bell Labs|Plan 9]] <!-- Supported OS -->
 |-
 |[[Kerrighed]]
 | {{Yes}} <!-- Process migration -->
 | {{Yes}} <!-- Process checkpointing -->
 | {{Yes}} <!-- Single process space -->
 | {{Yes}} <!-- Single root -->
 | {{Unk}} <!-- Single I/O space -->
 | {{Yes}} <!-- Single IPC space -->
 | {{Unk}} <!-- Cluster IP address -->
 | {{Yes|Open}} <!-- Source Model -->
 | {{No|{{dts|2010|06|14}}}} <!-- Latest release date -->
 | [[Linux]] 2.6.30 <!-- Supported OS -->
 |-
 |[[LinuxPMI]]<ref group="t">[[LinuxPMI]] is a successor to [[openMosix]]</ref>
 | {{Yes}} <!-- Process migration -->
 | {{Yes}} <!-- Process checkpointing -->
 | {{No}} <!-- Single process space -->
 | {{Yes}} <!-- Single root -->
 | {{No}} <!-- Single I/O space -->
 | {{No}} <!-- Single IPC space -->
 | {{Unk}} <!-- Cluster IP address -->
 | {{Yes|Open}} <!-- Source Model -->
 | {{No|{{dts|2006|06|18}}}} <!-- Latest release date -->
 | [[Linux]] 2.6.17 <!-- Supported OS -->
 |-
 |[[LOCUS (operating system)|LOCUS]]<ref group="t">[[LOCUS (operating system)|LOCUS]] was used to create [[IBM]] [[AIX]] [[Transparent Computing Facility|TCF]]</ref>
 | {{Yes}} <!-- Process migration -->
 | {{Unk}} <!-- Process checkpointing -->
 | {{Yes}} <!-- Single process space -->
 | {{Yes}} <!-- Single root -->
 | {{Yes}} <!-- Single I/O space -->
 | {{Yes|Yes<ref group="t">[[LOCUS (operating system)|LOCUS]] used [[named pipes]] for IPC</ref>}} <!-- Single IPC space -->
 | {{Unk}} <!-- Cluster IP address -->
 | {{No|Closed}} <!-- Source Model -->
 | {{No|{{dts|1988}}}} <!-- Latest release date -->
 | Native <!-- Supported OS -->
 |-
 |[[MOSIX]]
 | {{Yes}} <!-- Process migration -->
 | {{Yes}} <!-- Process checkpointing -->
 | {{No}} <!-- Single process space -->
 | {{Yes}} <!-- Single root -->
 | {{No}} <!-- Single I/O space -->
 | {{No}} <!-- Single IPC space -->
 | {{Unk}} <!-- Cluster IP address -->
 | {{No|Closed}} <!-- Source Model -->
 | {{Yes|{{dts|2017|10|24}}}} <!-- Latest release date -->
 | [[Linux]] <!-- Supported OS -->
 |-
 |[[openMosix]]<ref group="t">[[openMosix]] was a fork of MOSIX</ref>
 | {{Yes}} <!-- Process migration -->
 | {{Yes}} <!-- Process checkpointing -->
 | {{No}} <!-- Single process space -->
 | {{Yes}} <!-- Single root -->
 | {{No}} <!-- Single I/O space -->
 | {{No}} <!-- Single IPC space -->
 | {{Unk}} <!-- Cluster IP address -->
 | {{Yes|Open}} <!-- Source Model -->
 | {{No|{{dts|2004|12|10}}}} <!-- Latest release date -->
 | [[Linux]] 2.4.26 <!-- Supported OS -->
 |-
 |[[Open-Sharedroot]]<ref group="t">[[Open-Sharedroot]] is a shared root Cluster from ATIX</ref>
 | {{No}} <!-- Process migration -->
 | {{No}} <!-- Process checkpointing -->
 | {{No}} <!-- Single process space -->
 | {{Yes}} <!-- Single root -->
 | {{No}} <!-- Single I/O space -->
 | {{No}} <!-- Single IPC space -->
 | {{Yes}} <!-- Cluster IP address -->
 | {{Yes|Open}} <!-- Source Model -->
 | {{No|{{dts|2011|09|01}}}}<ref>{{cite web|url=https://github.com/Open-Sharedroot?tab=repositories |title=Open-Sharedroot GitHub repository|website=[[GitHub]]}}</ref> <!-- Latest release date -->
 | [[Linux]] <!-- Supported OS -->
 |-
 |[[OpenSSI]]
 | {{Yes}} <!-- Process migration -->
 | {{No}} <!-- Process checkpointing -->
 | {{Yes}} <!-- Single process space -->
 | {{Yes}} <!-- Single root -->
 | {{Yes}} <!-- Single I/O space -->
 | {{Yes}} <!-- Single IPC space -->
 | {{Yes}} <!-- Cluster IP address -->
 | {{Yes|Open}} <!-- Source Model -->
 | {{No|{{dts|2010|02|18}}}} <!-- Latest release date -->
 | [[Linux]] 2.6.10 ([[Debian]], [[Fedora (operating system)|Fedora]]) <!-- Supported OS -->
 |-
 |[[Plan 9 from Bell Labs|Plan 9]]
 | {{No}}<ref>{{Citation
  | last1 = Pike | first1 = Rob
  | last2 = Presotto | first2 = Dave
  | last3 = Thompson | first3 = Ken
  | last4 = Trickey | first4 = Howard
  | contribution = Plan 9 from Bell Labs
  | series = In Proceedings of the Summer 1990 UKUUG Conference
  | pages = 8
  | year = 1990
  | quote = Process migration is also deliberately absent from Plan 9.
  }}</ref> <!-- Process migration -->
 | {{No}} <!-- Process checkpointing -->
 | {{No}} <!-- Single process space -->
 | {{Yes}} <!-- Single root -->
 | {{Yes}} <!-- Single I/O space -->
 | {{Yes}} <!-- Single IPC space -->
 | {{Yes}} <!-- Cluster IP address -->
 | {{Yes|Open}} <!-- Source Model -->
 | {{Yes|{{dts|2015|01|09}}}} <!-- Latest release date -->
 | Native <!-- Supported OS -->
 |-
 |[[Sprite operating system|Sprite]]
 | {{Yes}} <!-- Process migration -->
 | {{Unk}} <!-- Process checkpointing -->
 | {{No}} <!-- Single process space -->
 | {{Yes}} <!-- Single root -->
 | {{Yes}} <!-- Single I/O space -->
 | {{No}} <!-- Single IPC space -->
 | {{Unk}} <!-- Cluster IP address -->
 | {{Yes|Open}} <!-- Source Model -->
 | {{No|{{dts|1992}}}} <!-- Latest release date -->
 | Native <!-- Supported OS -->
 |-
 |[[TidalScale]]
 | {{Yes}}<!-- Process migration -->
 | {{No}} <!-- Process checkpointing -->
 | {{Yes}} <!-- Single process space -->
 | {{Yes}} <!-- Single root -->
 | {{Yes}} <!-- Single I/O space -->
 | {{Yes}} <!-- Single IPC space -->
 | {{Yes}} <!-- Cluster IP address -->
 | {{No|Closed}} <!-- Source Model -->
 | {{Yes|{{dts|2020|08|17}}}} <!-- Latest release date -->
 | [[Linux]], [[FreeBSD]] <!-- Supported OS -->
 |-
 |[[TruCluster]]
 | {{No}} <!-- Process migration -->
 | {{Unk}} <!-- Process checkpointing -->
 | {{No}} <!-- Single process space -->
 | {{Yes}} <!-- Single root -->
 | {{No}} <!-- Single I/O space -->
 | {{No}} <!-- Single IPC space -->
 | {{Yes}} <!-- Cluster IP address -->
 | {{No|Closed}} <!-- Source Model -->
 | {{No|{{dts|2010|10|1}}}} <!-- Latest release date -->
 | [[Tru64]] <!-- Supported OS -->
 |-
 |[[VMScluster]]
 | {{No}} <!-- Process migration -->
 | {{No}} <!-- Process checkpointing -->
 | {{Yes}} <!-- Single process space -->
 | {{Yes}} <!-- Single root -->
 | {{Yes}} <!-- Single I/O space -->
 | {{Yes}} <!-- Single IPC space -->
 | {{Yes}} <!-- Cluster IP address -->
 | {{No|Closed}} <!-- Source Model -->
 | {{Yes|{{dts|2024|11|20}}}} <!-- Latest release date -->
 | [[OpenVMS]] <!-- Supported OS -->
 |-
 |[[z/VM]]
 | {{Yes}} <!-- Process migration -->
 | {{No}} <!-- Process checkpointing -->
 | {{Yes}} <!-- Single process space -->
 | {{No}} <!-- Single root -->
 | {{No}} <!-- Single I/O space -->
 | {{Yes}} <!-- Single IPC space -->
 | {{Unk}} <!-- Cluster IP address -->
 | {{No|Closed}} <!-- Source Model -->
 | {{Yes|{{dts|2022|09|16}}}} <!-- Latest release date -->
 | Native <!-- Supported OS -->
 |-
 |[[UnixWare NonStop Clusters]]<ref group="t">[[UnixWare NonStop Clusters]] was a base for [[OpenSSI]]</ref>
 | {{Yes}} <!-- Process migration -->
 | {{No}} <!-- Process checkpointing -->
 | {{Yes}} <!-- Single process space -->
 | {{Yes}} <!-- Single root -->
 | {{Yes}} <!-- Single I/O space -->
 | {{Yes}} <!-- Single IPC space -->
 | {{Yes}} <!-- Cluster IP address -->
 | {{No|Closed}} <!-- Source Model -->
 | {{No|{{dts|2000|6|}}}} <!-- Latest release date -->
 | [[UnixWare]] <!-- Supported OS -->
|} 
<references group="t"/>

==See also==
* [[Diskless shared-root cluster]]
* [[Distributed lock manager]]
* [[Distributed cache]]
* [[Parallel Virtual Machine]] - multiple system image alternative
* [[Message Passing Interface]] - multiple system image alternative

==Notes==
<references group=note/>

==References==
{{Reflist}}

{{DEFAULTSORT:Single System Image}}
[[Category:Cluster computing]]