Hot and Cold Migrations

1. Cold Migration :
A powered down Virtual Machine is carried to separate host or data store. Virtual Machine’s power state is OFF and there is no need of common shared storage. There is a lack of CPU check and there is long shortage time. Log files and configuration files are migrated from the source host to the destination host.

The first host’s Virtual Machine is shut down and again started on next host. Applications and OS are terminated on Virtual Machines before moving them to physical devices. User is given choice of movement of disks associated from one data store to another one.

2. Hot Migrations :
A powered on Virtual Machine is moved from one physical host to another physical host. A source host state is cloned to destination host and then that source host state is discarded. Complete state is shifted to the destination host. Network is moved to destination Virtual Machine.

A common shared storage is needed and CPU checks are put into use. Shortage time is very little. Without stoppage of OS or applications, they are shifted from Virtual Machines to physical machines. The physical server is freed for maintenance purposes and workloads (which are among physical servers) are dynamically balanced so as to run at optimized levels. Downtime of clients is easily avoidable.

Suspend first host’s Virtual Machine and then clone it across registers of CPU and RAM and again resume some time later on second host. This migration runs when source system is operative.

• Stage-0:
  Is Pre-Migration stage having functional Virtual Machine on primary host.
• Stage-1:
  Is Reservation stage initializing container on destination host.
• Stage-2:
  Is Iterative pre-copy stage where shadow paging is enabled and all dirty pages are cloned in succession rounds.
• Stage-3:
  Is Stop and copy where first host’s Virtual Machine is suspended and all remaining Virtual Machine state are synchronized on second host.
• Stage-4:
  Is Commitment where there is minimization of Virtual Machine state on first host.
• Stage-5:
  Is Activation stage where second host’s Virtual Machine start and establishes connection to all local computers resuming all normal activities.