| Summary: | 碩士 === 淡江大學 === 電機工程學系碩士班 === 104 === The environment of cloud service often consists of many virtual machines, however, in the case of a computer which is composed of multiple virtual machines, some inevitable errors of computer hardware or software would occur, resulting in damage to the system. Accordingly, the providers of cloud service must make a backup of virtual systems. The general system backup must be run under the condition of system down, which cannot be allowed in the cloud service. In order to let the virtualization system to backup at any time without affecting its service, a lot of snapshot techniques have been proposed to allow the virtualization system to backup without downtime.
The snapshot of virtualization system contains backup of hard disk and backup of state of system, when a system failure or other situations, it can restore the system to checkpoint by backup file which was generated by snapshot. In Xen, the snapshot of hard disk is depending on the Linux system’s Logical Volume Manager, and the state snapshot is depending Xen’s “save” command. Save command will shut the virtual machine down and generate the image file of state of the virtual machine, and the generation time of image file is based on memory size of the virtual machine, which is approximately several tens of seconds. For cloud services, such a downtime is not allowed.
In the research of snapshots in distributed systems, VNsnap finds that the snapshot of Xen virtual machine state causes a long downtime, so they propose a state snapshot mechanism of the virtual machine by modifying Xen’s migration function, and a host plays the target host to receive and store the state snapshot files of the virtual machine to achieve the purpose of reducing downtime. Because this method is modified from live migration, we call this method of “live-migrated save”.
This paper proposes another snapshot mechanism of state of virtual machine. By modifying Xen’s “save” function, it could make the real time backup of virtual machine state to the local host. Unlike “live-migrated save”, it does not demand daemon to receive data, so wasting of the resources of network bandwidth due to continuous transfer of memory data through the Internet will not occur.
This paper compares three methods, Xen’s “save”, “live-migrated save”, and the method which is proposed in this paper, and also compares the “live-migrated save” with the Xen’s migration, to point out their differences. During the snapshot, domU continuously writes data to its memory to make memory dirty, and we set the domU’s CPU cap by Credit Scheduler to produce different degrees of dirty frequency. Then, the command line parameter “-vvv” allows the system to display more detailed message to get the data of downtime, the execution time, and the amount of data transferred. Finally, experimental and simulation results prove that our method can save the cost of transmission under the condition that our downtime and execution time are similar to live-migrated save’s.
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