openstack-manuals/doc/ops-guide/source/arch-example-nova-network.rst

Example Architecture — Legacy Networking (nova)

This particular example architecture has been upgraded from Grizzly to Havana and tested in production environments where many public IP addresses are available for assignment to multiple instances. You can find a second example architecture that uses OpenStack Networking (neutron) after this section. Each example offers high availability, meaning that if a particular node goes down, another node with the same configuration can take over the tasks so that the services continue to be available.

Overview

The simplest architecture you can build upon for Compute has a single cloud controller and multiple compute nodes. The simplest architecture for Object Storage has five nodes: one for identifying users and proxying requests to the API, then four for storage itself to provide enough replication for eventual consistency. This example architecture does not dictate a particular number of nodes, but shows the thinking and considerations that went into choosing this architecture including the features offered.

Components

Component	Details
OpenStack release	Havana
Host operating system	Ubuntu 12.04 LTS or Red Hat Enterprise Linux 6.5, including derivatives such as CentOS and Scientific Linux
OpenStack package repository	Ubuntu Cloud Archive or RDO
Hypervisor	KVM
Database	MySQL*
Message queue	RabbitMQ for Ubuntu; Qpid for Red Hat Enterprise Linux and derivatives
Networking service	nova-network
Network manager	FlatDHCP
Single nova-network or multi-host?	multi-host*
Image service (glance) back end	file
Identity (keystone) driver	SQL
Block Storage (cinder) back end	LVM/iSCSI
Live Migration back end	Shared storage using NFS*
Object storage	OpenStack Object Storage (swift)

An asterisk (*) indicates when the example architecture deviates from the settings of a default installation. We'll offer explanations for those deviations next.

Note

The following features of OpenStack are supported by the example architecture documented in this guide, but are optional:

• Dashboard <Dashboard (horizon)>: You probably want to offer a dashboard, but your users may be more interested in API access only.
• Block storage <Block Storage service (cinder)>: You don't have to offer users block storage if their use case only needs ephemeral storage on compute nodes, for example.
• Floating IP address <floating IP address>: Floating IP addresses are public IP addresses that you allocate from a predefined pool to assign to virtual machines at launch. Floating IP address ensure that the public IP address is available whenever an instance is booted. Not every organization can offer thousands of public floating IP addresses for thousands of instances, so this feature is considered optional.
• Live migration <live migration>: If you need to move running virtual machine instances from one host to another with little or no service interruption, you would enable live migration, but it is considered optional.
• Object storage <Object Storage service (swift)>: You may choose to store machine images on a file system rather than in object storage if you do not have the extra hardware for the required replication and redundancy that OpenStack Object Storage offers.

Rationale

This example architecture has been selected based on the current default feature set of OpenStack Havana, with an emphasis on stability. We believe that many clouds that currently run OpenStack in production have made similar choices.

You must first choose the operating system that runs on all of the physical nodes. While OpenStack is supported on several distributions of Linux, we used Ubuntu 12.04 LTS (Long Term Support), which is used by the majority of the development community, has feature completeness compared with other distributions and has clear future support plans.

We recommend that you do not use the default Ubuntu OpenStack install packages and instead use the Ubuntu Cloud Archive. The Cloud Archive is a package repository supported by Canonical that allows you to upgrade to future OpenStack releases while remaining on Ubuntu 12.04.

KVM as a hypervisor complements the choice of Ubuntu—being a matched pair in terms of support, and also because of the significant degree of attention it garners from the OpenStack development community (including the authors, who mostly use KVM). It is also feature complete, free from licensing charges and restrictions.

MySQL follows a similar trend. Despite its recent change of ownership, this database is the most tested for use with OpenStack and is heavily documented. We deviate from the default database, SQLite, because SQLite is not an appropriate database for production usage.

The choice of RabbitMQ over other AMQP <Advanced Message Queuing Protocol (AMQP)> compatible options that are gaining support in OpenStack, such as ZeroMQ and Qpid, is due to its ease of use and significant testing in production. It also is the only option that supports features such as Compute cells. We recommend clustering with RabbitMQ, as it is an integral component of the system and fairly simple to implement due to its inbuilt nature.

As discussed in previous chapters, there are several options for networking in OpenStack Compute. We recommend FlatDHCP and to use Multi-Host networking mode for high availability, running one nova-network daemon per OpenStack compute host. This provides a robust mechanism for ensuring network interruptions are isolated to individual compute hosts, and allows for the direct use of hardware network gateways.

Live Migration is supported by way of shared storage, with NFS as the distributed file system.

Acknowledging that many small-scale deployments see running Object Storage just for the storage of virtual machine images as too costly, we opted for the file back end in the OpenStack Image service (Glance). If your cloud will include Object Storage, you can easily add it as a back end.

We chose the SQL back end for Identity over others, such as LDAP. This back end is simple to install and is robust. The authors acknowledge that many installations want to bind with existing directory services and caution careful understanding of the array of options available.

Block Storage (cinder) is installed natively on external storage nodes and uses the LVM/iSCSI plug-in. Most Block Storage plug-ins are tied to particular vendor products and implementations limiting their use to consumers of those hardware platforms, but LVM/iSCSI is robust and stable on commodity hardware.

While the cloud can be run without the OpenStack Dashboard, we consider it to be indispensable, not just for user interaction with the cloud, but also as a tool for operators. Additionally, the dashboard's use of Django makes it a flexible framework for extension.

Why not use OpenStack Networking?

This example architecture does not use OpenStack Networking, because it does not yet support multi-host networking and our organizations (university, government) have access to a large range of publicly-accessible IPv4 addresses.

Why use multi-host networking?

In a default OpenStack deployment, there is a single nova-network service that runs within the cloud (usually on the cloud controller) that provides services such as Network Address Translation (NAT), DHCP <Dynamic Host Configuration Protocol (DHCP)>, and DNS <Domain Name System (DNS)> to the guest instances. If the single node that runs the nova-network service goes down, you cannot access your instances, and the instances cannot access the Internet. The single node that runs the nova-network service can become a bottleneck if excessive network traffic comes in and goes out of the cloud.

Tip

Multi-host is a high-availability option for the network configuration, where the nova-network service is run on every compute node instead of running on only a single node.

Detailed Description

The reference architecture consists of multiple compute nodes, a cloud controller, an external NFS storage server for instance storage, and an OpenStack Block Storage server for volume storage. A network time service (Network Time Protocol (NTP)) synchronizes time on all the nodes. FlatDHCPManager in multi-host mode is used for the networking. A logical diagram for this example architecture shows which services are running on each node:

The cloud controller runs the dashboard, the API services, the database (MySQL), a message queue server (RabbitMQ), the scheduler for choosing compute resources (nova-scheduler), Identity services (keystone, nova-consoleauth), Image services (glance-api, glance-registry), services for console access of guests, and Block Storage services, including the scheduler for storage resources (cinder-api and cinder-scheduler).

Compute nodes are where the computing resources are held, and in our example architecture, they run the hypervisor (KVM), libvirt (the driver for the hypervisor, which enables live migration from node to node), nova-compute, nova-api-metadata (generally only used when running in multi-host mode, it retrieves instance-specific metadata), nova-vncproxy, and nova-network.

The network consists of two switches, one for the management or private traffic, and one that covers public access, including floating IPs. To support this, the cloud controller and the compute nodes have two network cards. The OpenStack Block Storage and NFS storage servers only need to access the private network and therefore only need one network card, but multiple cards run in a bonded configuration are recommended if possible. Floating IP access is direct to the Internet, whereas Flat IP access goes through a NAT. To envision the network traffic, use this diagram:

Optional Extensions

You can extend this reference architecture as follows:

• Add additional cloud controllers (see ops-maintenance).
• Add an OpenStack Storage service (see the Object Storage chapter in the Installation Tutorials and Guides for your distribution).
• Add additional OpenStack Block Storage hosts (see ops-maintenance).