Synnefo » snf-ganeti

Virtual cluster support

=======================

Documents Ganeti version 2.11

.. contents::

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 :doc:`design document <design-virtual-clusters>`.

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 :ref:`limitations <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:

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``.

.. _vcluster-setup:

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.

.. TODO: Describe setup of non-root virtual cluster

Use

---

Once the virtual cluster has been :ref:`set up <vcluster-setup>`, 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:

.. highlight:: shell-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):

.. highlight:: shell-example

::

  $ 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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