In recent years, as a virtual machine producing method, Linked Clone/Differential Clone system has been developed which share a master image. This system is suitable for a thin client system in which a plurality of users share one virtual machine because an extremely smaller disk space is required for producing a large number of virtual machines of the same image as compared with a common system.
In the Linked Clone/Differential Clone system, since use of each virtual machine generates a difference from a master image to consume a disk capacity, a differential disk should be deleted periodically to execute reverting processing of initializing a virtual machine.
As an existing method of executing maintenance of a thin client system, proposed is a method in which with a virtual machine duplexed, one side executes updating and a user uses the other side virtual machine to restore an image after the use by the user.
The thin client system which shares a virtual machine has a plurality of features as follows.
First feature is allowing a user to use any virtual machine as long as it is based on the same image. Second feature is allowing execution of reverting processing when not used by a user. Third feature is allowing reverting processing any time because no information peculiar to a user is preserved in a virtual machine.    Patent Literature 1: Japanese Patent Laying-Open No. 2011-248742    Patent Literature 2: Japanese Patent Laying-Open No. 2010-257426    Patent Literature 3: Japanese Patent Laying-Open No. 2010-231661
A large scale thin client system having several thousands to several tens of thousands of virtual machines to be reverted has a problem of being difficulty in production of a standby system by simple virtual machine duplexing or in execution of maintenance of all the machines in the lump at a fixed maintenance time.
It is necessary to provide a method of efficiently executing reverting processing in a thin client system by making the most of these features of the system in which a virtual machine is shared among users.