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Global Snapshot of a Distributed System Running on Virtual Machines

Recently, a new concept called desktop cloud emerged, which was developed to offer cloud computing services on non-dedicated resources. Similarly to cloud computing, desktop clouds are based on virtualization, and like other computational systems, may experience faults at any time. As a consequence, reliability has become a concern for researchers. Fault-tolerance strategies focused on independent virtual machines include snapshots (checkpoints) to resume the execution from a healthy state of a virtual machine on the same or another host, which is trivial because hypervisors provide this function. However, it is not trivial to obtain a global snapshot of a distributed system formed by applications that communicate among them because the concept of global clock does not exist, so it can not be guaranteed that snapshots of each VM will be taken at the same time. Therefore, some protocol is needed to coordinate the participants to obtain a global snapshot. In this paper, we propose a global snapshot protocol called UnaCloud Snapshot for its application in the context of desktop clouds over TCP/IP networks. That differs from other proposals that use a virtual network to inspect and manipulate the traffic circulating among virtual machines making it difficult to apply them to more realistic environments. We obtain a consistent global snapshot for a general distributed system running on virtual machines that maintains the semantics of the system without modifying applications running on virtual machines or hypervisors. A first prototype was developed and the preliminary results of our evaluation are presented.