Lec 19: VM Placement and VM Migration

Overview

• Topic: VM placement (choosing hosts for virtual machines) and VM migration (moving VMs between hosts).
• Purpose: maximize resource utilization and performance, minimize energy use and costs, and reduce SLA violations, latency, congestion, and failures.

VM placement

VM placement: selecting the most suitable physical host from a pool of servers based on VM requirements (CPU, RAM, network).

Goals:

• Energy efficiency
• Load balancing
• Minimize SLA and network issues
• Cost reduction

Placement schemes:

• Static placement
  • VM-to-host mapping is fixed for the VM’s lifetime (no host changes).
• Dynamic placement
  • VM host can change over time based on conditions.
  • Reactive: migrate only after problems appear (e.g., overload, maintenance, power issues).
  • Proactive: migrate before problems occur based on predictions or thresholds.

VM migration

VM migration: moving a VM (or its storage) from one physical host, storage location, or data center to another to optimize resources, enable maintenance, or balance load.

Two main migration types:

Hot (live) migration

• VM keeps running during most of the process; the goal is minimal downtime (usually milliseconds to seconds).
• Typical requirements:
  • Shared storage between source and target hosts.
  • CPU compatibility checks between hosts.
• Overhead:
  • Iterative memory copying can cause repeated copying of pages that get dirtied during transfer.
• Typical process stages (often numbered 0–5):
  1. Pre-migration: select target host, mirror block devices, ensure resources are available.
  2. Reservation: initialize container on target and reserve CPU/RAM/storage.
  3. Iterative pre-copy: iteratively copy memory pages from source to target (shadow paging; repeated rounds for pages dirtied during copy).
  4. Stop-and-copy: briefly suspend the VM on the source, copy remaining CPU/memory state, and update ARP/network to route to target (small downtime).
  5. Commitment: release resources on the source host and delete the old instance.
  6. Activation: start/resume the VM on the target and reconnect devices/network; users continue.

Cold (powered-off) migration

• The VM is stopped; its entire state (disk, RAM image, config, logs) is moved, then the VM is started on the target.
• Does not require shared storage or the same CPU compatibility checks required by live migration.
• Simpler process but incurs full downtime for the duration of the move.
• Useful for moving associated disks between datastores and across data centers.

Key comparisons

• Live (hot) migration
  • Pros: lower downtime.
  • Cons: needs shared storage and CPU compatibility checks; more complex due to iterative copying and coordination.
• Cold migration
  • Pros: simpler; can move disks and cross data centers without shared storage.
  • Cons: higher downtime.

Type of content

This is a tutorial/lecture covering definitions, goals, placement schemes, migration methods, detailed live-migration steps, and practical requirements/overheads.

Source

Course lecture “Lec 19: VM Placement and VM Migration” — presented by the video’s instructor (unnamed).