Concepts

Concepts To use the OpenStack Compute API effectively, you should understand several key concepts: Server A virtual machine (VM) instance in the compute system. Flavor and image are requisite elements when creating a server. Flavor An available hardware configuration for a server. Each flavor has a unique combination of disk space, memory capacity and priority for CPU time. Image A collection of files used to create or rebuild a server. Operators provide a number of pre-built OS images by default. You may also create custom images from cloud servers you have launched. These custom images are useful for backup purposes or for producing “gold” server images if you plan to deploy a particular server configuration frequently. Reboot Use this function to perform either a soft or hard reboot of a server. With a soft reboot, the operating system is signaled to restart, which allows for a graceful shutdown of all processes. A hard reboot is the equivalent of power cycling the server. The virtualization platform should ensure that the reboot action has completed successfully even in cases in which the underlying domain/VM is paused or halted/stopped. Rebuild Use this function to remove all data on the server and replaces it with the specified image. Server ID and IP addresses remain the same. Resize Use this function to convert an existing server to a different flavor, in essence, scaling the server up or down. The original server is saved for a period of time to allow rollback if there is a problem. All resizes should be tested and explicitly confirmed, at which time the original server is removed. All resizes are automatically confirmed after 24 hours if you do not confirm or revert them. Pause You can pause a server by making a pause request. This request stores the state of the VM in RAM. A paused instance continues to run in a frozen state. Suspend Administrative users might want to suspend an instance if it is infrequently used or to perform system maintenance. When you suspend an instance, its VM state is stored on disk, all memory is written to disk, and the virtual machine is stopped. Suspending an instance is similar to placing a device in hibernation; memory and vCPUs become available to create other instances.