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To create a template, an administrator creates and customizes a virtual machine. Desired packages are installed, customized configurations are applied, the virtual machine is prepared for its intended purpose in order to minimize the changes that must be made to it after deployment. An optional but recommended step before creating a template from a virtual machine is generalization. Generalization is used to remove details like system user names, passwords, and timezone information that will change upon deployment. Generalization does not affect customized configurations. Generalization of Windows and Linux guests in the Red Hat Virtualization environment is discussed further in Templates in the Virtual Machine Management Guide. Red Hat Enterprise Linux guests are generalized using sys-unconfig. Windows guests are generalized using sys-prep.
When the virtual machine that provides the basis for a template is satisfactorily configured, generalized if desired, and stopped, an administrator can create a template from the virtual machine. Creating a template from a virtual machine causes a read only copy of the specially configured virtual disk image to be created. The read only image will form the backing image for all subsequently created virtual machines that are based on that template. In other words, a template is essentially a customized read only disk image with an associated virtual hardware configuration. The hardware can be changed in virtual machines created from a template, for instance provisioning two gigabytes of RAM for a virtual machine created from a template that has one gigabyte of RAM. The template disk image, however, can not be changed as doing so would result in changes for all virtual machines based on the template.
When a template has been created, it can be used as the basis for multiple virtual machines. Virtual machines are created from a given template using a Thin provisioning method or a Clone provisioning method. Virtual machines that are cloned from templates take a complete writable copy of the template base image, sacrificing the space savings of a the thin creation method in exchange for no longer depending on the presence of the template. Virtual machines that are created from a template using the thin method use the read only image from the template as a base image, requiring that the template and all virtual machines created from it be stored on the same storage domain. Changes to data and newly generated data are stored in a copy on write image. Each virtual machine based on a template uses the same base read only image, as well as a copy on write image that is unique to the virtual machine. This provides storage savings by limiting the number of times identical data is kept in storage. Furthermore, frequent use of the read only backing image can cause the data being accessed to be cached, resulting in a net performance increase.