Hosts & Clusters 3.4

What is a Cluster

Clusters are pools of hosts that share datastores and virtual networks. Clusters are used for load balancing, high availability, and high performance computing.

What is a Host

A Host is a server that has the ability to run Virtual Machines and that is connected to OpenNebula's Frontend server. OpenNebula can work with Hosts with a heterogeneous configuration, i.e. you can connect Hosts to the same OpenNebula with different hypervisors or Linux distributions as long as these requirements are fulfilled:

  • Every Host need to have a oneadmin account.
  • OpenNebula's Frontend and all the Hosts need to be able to resolve, either by DNS or by /etc/hosts the names of all the other Hosts and Frontend.
  • The oneadmin account in any Host or the Frontend should be able to ssh passwordlessly to any other Host or Frontend. This is achieved either by sharing the $HOME of oneadmin accross all the servers with NFS or by manually copying the ~/.ssh directory.
  • It needs to have a hypervisor supported by OpenNebula installed and properly configured. The correct way to achieve this is to follow the specific guide for each hypervisor.
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Overview of Components

There are three components regarding Hosts:

  • Host Management: Host management is achieved through the onehost CLI command or through the Sunstone GUI. You can read about Host Management in more detail in the Managing Hosts guide.
  • Cluster Management: Hosts can be grouped in Clusters. These Clusters are managed with the onecluster CLI command, or through the Sunstone GUI. You can read about Cluster Management in more detail in the Managing Clusters guide.
  • Host Monitorization: In order to keep track of the available resources in the Hosts, OpenNebula launches a Host Monitoring driver, called IM (Information Driver), which gathers all the required information and submits it to the Core. The default IM driver executes ssh commands in the host, but other mechanism are possible, name the Ganglia Monitoring System. There is further information on this topic in the Monitoring Subsystem guide.