How vMotion works

1.       The virtual machine’s memory state is copied over the vMotion network from the source host to the target host. Users continue to access the virtual machine and, potentially, update pages in memory. A list of modified pages in memory is kept in a memory bitmap on the source host.

2.        After most of the virtual machine’s memory is copied from the source host to the target host, the virtual machine is quiesced. No additional activity occurs on the virtual machine. In the quiesce period, vMotion transfers the virtual machine device state and memory bitmap to the destination host.

3.       Immediately after the virtual machine is quiesced on the source host, the virtual machine is initialized and starts running on the target host. A Reverse Address Resolution Protocol (RARP) request notifies the subnet that virtual machine A’s MAC address is now on a new switch port.

4.        Users access the virtual machine on the target host instead of the source host.

5.       The memory pages that the virtual machine was using on the source host are marked as free.

6.       Shadow VM created on the destination host.

7.       Copy each memory page from the source to the destination via the vMotion network. This is known as preCopy.

8.       Perform another pass over the VM’s memory, copying any pages that changed during the last preCopy iteration.

9.        Stun the VM on the source and resume it on the destination.