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7.3. Pools

Virtual machine pools allow for rapid provisioning of numerous identical virtual machines to users as desktops. Users who have been granted permission to access and use virtual machines from a pool receive an available virtual machine based on their position in a queue of requests. Virtual machines in a pool do not allow data persistence; each time a virtual machine is assigned from a pool, it is allocated in its base state. This is ideally suited to be used in situations where user data is stored centrally.

Virtual machine pools are created from a template. Each virtual machine in a pool uses the same backing read-only image, and uses a temporary copy-on-write image to hold changed and newly generated data. Virtual machines in a pool are different from other virtual machines in that the copy-on-write layer that holds user-generated and -changed data is lost at shutdown. The implication of this is that a virtual machine pool requires no more storage than the template that backs it, plus some space for data generated or changed during use. Virtual machine pools are an efficient way to provide computing power to users for some tasks without the storage cost of providing each user with a dedicated virtual desktop.

Example 7.1. Example Pool Usage

A technical support company employs 10 help desk staff. However, only five are working at any given time. Instead of creating ten virtual machines, one for each help desk employee, a pool of five virtual machines can be created. Help desk employees allocate themselves a virtual machine at the beginning of their shift and return it to the pool at the end.