Explain how migration of memory, files, and network resources happen in cloud computing.

In cloud computing, VM migration is a process where a virtual machine is moved from one physical host to another. This allows better load balancing, hardware maintenance, and energy efficiency. During migration, three main components must be handled:

Memory Migration

• Goal: Move the VM’s memory from the source to the destination system.
• Challenge: VM memory can be several hundreds of MBs to a few GBs.
• Technique:
  • Internet Suspend-Resume (ISR):
    • Exploits temporal locality (only recently changed data is different).
    • Memory is treated as a tree of small subfiles.
    • Only modified subfiles are transmitted during migration.
  • Live Migration:
    • Uses pre-copy method.
    • First transfers all memory pages.
    • Then sends only modified (dirty) pages in iterations.
    • Final step transfers CPU state and last dirty pages.
    • Keeps downtime very low (few milliseconds to seconds).
  • Post-copy:
    • Transfers only once.
    • Higher downtime but lower total migration time.
  • Memory Compression:
    • Compress memory pages to reduce size.
    • Fast decompression on the destination system.

File System Migration

• Goal: Provide the same file system view to the VM on the new host.
• Problem: Large disks are difficult to transfer entirely.
• Solutions:
  • Virtual Disk Transfer:
    • Copy the entire virtual disk image, but slow and not scalable.
  • Global File System (like NFS):
    • Files are network-accessible, so no need to copy.
  • ISR with Smart Copying:
    • Uses spatial locality (users move between predictable places).
    • Only differences between source and destination file systems are transferred.
  • Proactive Transfer:
    • Predict where the VM will move next and preload common data.

Network Migration

• Goal: Maintain all active network connections during migration.
• Techniques:
  • Assign each VM a virtual IP address and virtual MAC address.
  • VMM maps these to physical machines.
  • Use Unsolicited ARP to notify other systems about the VM’s new location.
  • Optionally, keep the same MAC address and let the switch detect the move.
• Live Migration Support:
  • Protocol states are moved with the VM.
  • Applications stay connected without restarting.
  • Common in LAN or datacenter environments.

Example: Xen VM Live Migration

• Xen hypervisor allows VM migration using Domain 0 (Dom0).
• Dom0 handles migration tasks using send/receive model.
• Uses pre-copy strategy to move memory and CPU state.
• Ensures minimal downtime and keeps applications running.