Hyper-V Live VM Migration Procedure

1. Live migration setup occurs. 

During the live migration setup stage, the source server creates a connection with the destination server. This connection transfers the virtual machine configuration data to the destination server. A skeleton virtual machine is set up on the destination server and memory is allocated to the destination virtual machine.


2. Memory pages are transferred from the source node to the destination node

• In the second stage of a live migration, the memory assigned to the migrating virtual machine is copied over the network to the destination server. This memory is referred to as the "working set" of the migrating virtual machine. A page of memory is 4 KB.
• In addition to copying the working set to the destination server, Hyper-V monitors the pages in the working set on the source server. As memory pages are modified in the source server, they are tracked and marked as being modified. 
• During this phase of the migration, the migrating virtual machine continues to run. Hyper-V iterates the memory copy process several times, with each iteration requiring a small number of modified pages to be copied. 

3. Modified pages are transferred.

• This third phase of the migration is a memory copy process that duplicates the remaining modified memory pages to the destination server. The source server transfers the CPU and device state of the virtual machine to the destination server.
• During this stage, the network bandwidth available between the source and destination servers is critical to the speed of the live migration. Using a 1 Gigabit Ethernet or faster is important. The faster the source server transfers the modified pages from the migrating virtual machines working set, the more quickly the live migration is completed. 
• The number of pages transferred in this stage is determined by how actively the virtual machine accesses and modifies the memory pages. The more modified pages there are, the longer it takes to transfer all pages to the destination server. 

4. The storage handle is moved from the source server to the destination server

• During the fourth stage of a live migration, control of the storage such as any virtual hard disk files or physical storage attached through a virtual Fibre Channel adapter, is transferred to the destination server. 

5. The virtual machine is brought online on the destination server.

• In the fifth stage of a live migration, the destination server now has the up-to-date working set as well as access to any storage used by the virtual machine. At this point, the virtual machine is resumed. 

6. Network cleanup occurs. 

• In the final stage of a live migration, the migrated virtual machine is running on the destination server. At this point, a message is sent to the network switch. This message causes the network switch to obtain the new MAC addresses of the migrated virtual machine so that network traffic to and from the virtual machine can use the correct switch port. 

Reference
[1] Virtual Machine Live Migration Overview