Tracking modifications to a virtual machine image that occur during backup of the virtual machine

A method for virtual machine backup in a computer system, the computer system comprising a processor unit arranged to run a hypervisor running one or more virtual machines and to run multiple execution threads;

• a cache connected to the processor unit and comprising a plurality of cache rows, each cache row comprising a memory address, a cache line, and an image modification flag;
• and a memory connected to the cache;
• the method comprising:
• maintaining a thread mask flagging those threads that relate to one or more virtual machines being backed up;
• defining a log in the memory;
• setting the image modification flag for a cache line modified by a virtual machine being backed up by reference to the thread mask;
• and
• writing only the memory address of the flagged cache rows in the defined log.

A computer program product for virtual machine backup, the computer system comprising a processor unit arranged to run a hypervisor running one or more virtual machines and to run multiple execution threads;

• a cache connected to the processor unit and comprising a plurality of cache rows, each cache row comprising a memory address, a cache line, and an image modification flag;
• and a memory connected to the cache;
• the computer program product comprising a computer readable storage medium having program instructions embodied therewith, the program instructions executable by a computer to cause the computer to:
• maintain a thread mask flagging those threads that relate to one or more virtual machines being backed up;
• define a log in the memory;
• set the image modification flag for a cache line modified by a virtual machine being backed up by reference to the thread mask;
• and
• write only the memory address of the flagged cache rows in the defined log.