Glance is the image management component. Once we're authenticated, there are a few resources that need to be available for an instance to launch. The first resource we'll look at is the disk image to launch from. Before a server is useful, it needs to have an operating system installed on it. This is a boilerplate task that cloud computing has streamlined by creating a registry of pre-installed disk images to boot from. Glance serves as this registry within an OpenStack deployment. In preparation for an instance to launch, a copy of a selected Glance image is first cached to the compute node where the instance is being launched. Then, a copy is made to the ephemeral disk location of the new instance. Subsequent instances launched on the same compute node using the same disk image will use the cached copy of the Glance image.
The images stored in Glance are sometimes called sealed-disk images. These images are disk images that have had the operating system installed but have had things such as the Secure Shell (SSH) host key and network device MAC addresses removed. This makes the disk images generic, so they can be reused and launched repeatedly without the running copies conflicting with each other. To do this, the host-specific information is provided or generated at boot. The provided information is passed in through a post-boot configuration facility called cloud-init.
Usually, these images are downloaded from distribution's download pages. If you search the Internet for your favorite distribution's name and cloud image, you will probably get a link to where to download a generic pre-built copy of a Glance image, also known as a cloud image.
The images can also be customized for special purposes beyond a base operating system installation. If there was a specific purpose for which an instance would be launched many times, then some of the repetitive configuration tasks could be performed ahead of time and built into the disk image. For example, if a disk image was intended to be used to build a cluster of web servers, it would make sense to install a web server package on the disk image before it was used to launch an instance. It would save time and bandwidth to do it once before it is registered with Glance instead of doing this package installation and configuration over and over each time a web server instance is booted.
There are quite a few ways to build these disk images. The simplest way is to do a virtual machine installation manually, make sure that the host-specific information is removed, and include cloud-init in the built image. Cloud-init is packaged in most major distributions; you should be able to simply add it to a package list. There are also tools to make this happen in a more autonomous fashion. Some of the more popular tools are virt-install, Oz, and appliance-creator. The most important thing about building a cloud image for OpenStack is to make sure that cloud-init is installed. Cloud-init is a script that should run post boot to connect back to the metadata service.
In Chapter 3, Image Management, when Glance is covered in greater detail, we will download a pre-built image and use it to demonstrate how Glance works.