Computer cluster in virtual machines

Computer cluster in virtual machines

Computer clusters run usually on physical computers. With the virtualization approach there are new possibilities of setting up different kinds of clusters.

There are different categorizations of such clusters, depending

  • on the type of underlying virtualization, and
  • of the level, where some Cluster Manager is running.

This article currently describes only high-availability clusters.

Note that the implementation sections give only some typical examples of the described cases - this is not a list of all available configurations.

Contents

Definitions

Because there are different sets of terms meaning the same concept, here is a small list of definitions used in this article. Things not defined here, can be found in the Computer Clusters article.

Host System

The Host System is the hardware and the virtualization software, like a hypervisor.

Guest System

The Guest System is the operating system that is running atop a Host System. Sometimes this is called Virtual Machine, but mostly every vendor of virtualization technology has its own name for this.

Cluster Manager

The Cluster Manager as used in this article, is software handling all the things that are needed in a computer cluster environment, e. g. supervision of resources (processes, IP Addresses, ...), restarting of resources, handling of failover.

Cluster Service

A Cluster Service is a service (application) running under control of a Cluster Manager. This is sometimes also called Process Resource.

Virtual vs. Physical

Virtual Cluster Nodes on one Host System

All the cluster nodes [note: a definition for node is needed above as that term is not defined or linked - one can only assume that "node" is synonymous to "guest" as defined above when used in a cluster] are all running on the same Host System. This does not increase any availability of the applications in case of a hardware outage, therefore it is mostly only used during development and testing of a Cluster Service.

Example: LinuxHA on Xen

Different Guest Systems run on one Host System. The Host System uses Xen as virtualization technology. The Guest Systems use Linux-HA for the high availability.[1] [note: this example does not help in clarifying the concepts being defined and appears a gratuitous mention of a specific vendor]

Virtual Cluster Nodes on different Host Systems

All nodes of the cluster are running on different Host Systems. This gives the same availability of the underlying cluster technology in case of a hardware outage and also gives the possibility to live migrate one virtual cluster node to another Host System for maintenance of a Host System.

Virtual - Physical Cluster

This scenario only makes sense for high availability failover clusters. One node is running on the physical machine the other on a Guest System. The normal way of operation is, that everything runs on the physical machine, only when there is a failure, the node on the Guest System gets control. The advantage of this scenario is, that it is possible to run some failover nodes in some Guest Systems of the same Host System. So it is possible to save mostly all of the additional second nodes. The disadvantage is, that the virtualized computer may not handle the load, when all physical nodes stop working at the same time.

Cluster Manager Level

Cluster Manager only in Guest Systems

The Cluster Manager runs only on a Guest System level. Two (or more) Guest Systems run as a cluster.

Example: Solaris Cluster in Guest Systems

There is an experiment from SUN running Solaris Cluster atop VMware.[2]

Example: openMosix for Xen

It is possible to run different Guest Systems atop Xen as an openMosix cluster.[3]

Cluster Manager only on Host Systems

The Cluster Manager is running on the Host System level. It supervises the Guest Systems themselves and can react on some event: e. g. to restart the Guest System on another machine.

Example: VMware HA

The VMware HA can restart Guest Systems when they fail.[4]

Independent Cluster Manager on Guest and Host Systems

This is adds the features of the Guest Only and the Host Only Clustering.

Integrated Cluster Manager on Guest and Host System

The Cluster Manager is running in the Host and Guest System level and knows about resources (e.g. processes) inside a Guest System and also about whole Guest Systems. It can therefor react on problems inside a Guest System or even of a problem with a whole Guest System, when e.g. the Guest System hangs.

Benefits

For HA clusters, the benefit for using virtual nodes is that even during hardware maintenance, all cluster nodes stay available. This can be achieved live by migrating one Virtual Cluster Node from the Host System that must be maintained to some other Host System.

References

  1. ^ Maurer, Ryan: Xen Virtualization and Linux Clustering
  2. ^ Sun Cluster Oasis
  3. ^ openMosix, an Open Source Linux Cluster Project
  4. ^ VMware: Provide High Availability at a Lower Cost http://www.vmware.com/products/vi/vc/ha.html

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