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# Virtual cluster support

Documents Ganeti version 2.8

Contents

- Virtual cluster support
  - Introduction
  - Limitations
  - Basics
  - Setup
  - Use

## Introduction

This is a description of Ganeti’s support for virtual clusters introduced in version 2.7. The original design is described in a separate design document.

A virtual cluster consists of multiple virtual nodes (instances of Ganeti daemons) running on the same physical machine within one operating system. This way multiple (virtual) nodes can be simulated using a single machine. Virtual clusters can be run as a user without root privileges (see limitations).

While not implemented in the helper setup script at the time of this writing, virtual clusters can also be split over multiple physical machines, allowing for even more virtual nodes.

## Limitations

Due to historical and practical design decisions virtual clusters have several limitations.

- “fake” hypervisor only
- Instances must be diskless or file-backed
- Node information is the same over multiple virtual nodes (e.g. free memory)
- If running as a user without root privileges, certain operations are not available; some operations are not useful even when running as root (e.g. powercycle)
- OS definitions must be prepared for this setup
- Setup is partially manual, especially when not running as root

## Basics

Ganeti programs act as running on a virtual node if the environment variables `GANETI_ROOTDIR` and `GANETI_HOSTNAME` are set. The former must be an absolute path to a directory with the last component being equal to the value of `GANETI_HOSTNAME`, which contains the name of the virtual node. The reason for this requirement is that one virtual node must be able to compute an absolute path on another node for copying files via SSH.

The whole content of `GANETI_ROOTDIR` is the node directory, its parent directory (without hostname) is the cluster directory.

Example for environment variables:

    GANETI_ROOTDIR=/tmp/vcluster/node1.example.com
    GANETI_HOSTNAME=node1.example.com

With this example the node directory is `/tmp/vcluster/node1.example.com` and the cluster directory `/tmp/vcluster`.

## Setup

A script to configure virtual clusters is included with Ganeti as `tools/vcluster-setup` (usually installed as `/usr/lib/ganeti/tools/vcluster-setup`). Running it with the `-h` option prints a usage description. The script creates all necessary directories, configures network interfaces, adds or updates entries in `/etc/hosts` and generates a small number of helper scripts.

## Use

Once the virtual cluster has been set up, the cluster can be initialized. The instructions for doing so have been printed by the `vcluster-setup` script together with other useful information, such as the list of virtual nodes. The commands printed should be used to configure the list of enabled hypervisors and other settings.

To run commands for a specific virtual node, the script named `cmd` located in the node directory can be used. It takes a command as its argument(s), sets the environment variables `GANETI_ROOTDIR` and `GANETI_HOSTNAME` and then runs the command. Example:

    # Let's create a cluster with node1 as its master node
    $ cd /tmp/vcluster
    $ node1.example.com/cmd gnt-cluster info
    Cluster name: cluster.example.com
    …
    Master node: node1.example.com
    …
    # Configure cluster as per "vcluster-setup" script
    $ node1.example.com/cmd gnt-cluster modify …

Scripts are provided in the cluster root directory to start, stop or restart all daemons for all virtual nodes. These are named `start-all`, `stop-all` and `restart-all`. `ganeti-watcher` can be run for all virtual nodes using `watcher-all`.

Adding an instance (assuming node1.example.com is the master node as per the example above):

    $ node1.example.com/cmd gnt-instance add --os-size 1G \
      --disk-template=file --os-type dummy -B memory=192 -I hail \
      instance1.example.com

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