No matter how overused the word "cloud" has become as a buzzword, it has had little effect on the cloud's popularity, especially in the enterprise: OpenNebula, openQRM, and Eucalyptus are all examples of enterprise clouds. However, no solution has enjoyed the kind of attention received by OpenStack, which resulted from a collaboration of the US space agency NASA and the US hosting service provider Rackspace.

In the past two years, the project has evolved from the underdog to the standard; OpenStack conferences attract far more visitors than many traditional community events. Even the leaders of the OpenStack Foundation [1] are surprised by the success, and companies even evaluate their cloud solutions with a preference for OpenStack (see the box "OpenStack in the Enterprise").

The Touchy Topic of Storage

When admins plan to set up in the cloud, they must deal with the topic of storage. Traditional storage systems are not sufficient in cloud computing because of two factors: scalability and automation. An admin can include classic storage in automated processes, whereas the issue of scalability can be more difficult to manage: Once a SAN is in the rack, any potential expansion is often expensive – if it is even feasible.

One aggravating factor for a public cloud provider is that you can hardly predict the required disk space. On the basis of recent demand, you might be able to calculate the growth of disk space, but that is little help if a new customer appears out of the blue requiring 5TB of space for its image hosting service. As with computing components, the provider must be able to expand the cloud quickly, and this is where the open source and free Ceph software enters the game.

Ceph Rules

The source code of the distributed filesystem Ceph [3] is largely covered by the LGPL and BSD licenses. The software is currently considered a hot product in the storage scene because it unites precisely these capabilities.

Thanks to these features, Inktank, the provider of the storage solution, has been able to compete for market share with long-established companies like EMC and NetApp.

An existing Ceph cluster can be easily extended by adding disks; only commodity hardware is required. Normal SATA disks complete the array, which affects the project budget considerably less than a (usually much smaller) SAS disk would.

Opportunely, Inktank has promoted the seamless integration of OpenStack and Ceph in recent months and, in a sense, already ensures at the factory that OpenStack optimally leverages the capabilities of Ceph. To determine the ideal location for Ceph and OpenStack, it makes sense to look at the kind of data the OpenStack components process and how they deal with storage.

Who Stores What?

In OpenStack services, two types of data basically occur: metadata and the actual payload. Technically, the metadata includes the cloud-specific configuration, which is usually handled by a separate database, such as MySQL. Most of the services deployed for virtualization purposes in OpenStack exclusively store metadata in the long term; this includes Keystone (identity service), Neutron (Network as a Service), Heat (Orchestration), and Ceilometer (monitoring).

The OpenStack dashboard, a.k.a. Horizon, does not create any data – either meta- or user. The compute service Nova is a special case: The component takes on the job of managing virtual machines. In addition to persistent metadata, it creates temporary images of virtual machines. The images of VMs in OpenStack are volatile, virtually by definition: In technical jargon, this is known as "ephemeral storage." If Nova wants to store a VM permanently, it needs to rely on an additional service.

This just leaves two services, Glance and Cinder, which create payload data in addition to their own metadata. Glance is the image service. Its purpose in life is to offer redundant image files from which new VMs can be booted. Cinder is the block storage service; it steps in to help Nova when the VMs require persistent storage. The task is thus to team Ceph with these two services to achieve optimal OpenStack integration.