If you need to take a host offline for maintenance, you can move the virtual machine to another host. Migration with vMotion™ allows virtual machine processes to continue working throughout a migration.
With vMotion, you can change the host on which a virtual machine is running, or you can change both the host and the datastore of the virtual machine.
When you migrate virtual machines with vMotion and choose to change only the host, the entire state of the virtual machine is moved to the new host. The associated virtual disk remains in the same location on storage that is shared between the two hosts.
When you choose to change both the host and the datastore, the virtual machine state is moved to a new host and the virtual disk is moved to another datastore. vMotion migration to another host and datastore is possible in vSphere environments without shared storage.
After the virtual machine state is migrated to the alternate host, the virtual machine runs on the new host. Migrations with vMotion are completely transparent to the running virtual machine.
The state information includes the current memory content and all the information that defines and identifies the virtual machine. The memory content includes transaction data and the bits of the operating system and applications that are in the memory. The defining and identification information stored in the state includes all the data that maps to the virtual machine hardware elements, such as BIOS, devices, CPU, MAC addresses for the Ethernet cards, chip set states, registers, and so forth.
When you migrate a virtual machine with vMotion, the new host for the virtual machine must meet compatibility requirements so that the migration can proceed.
Migration with vMotion occurs in three stages:
When the migration with vMotion is requested, vCenter Server verifies that the existing virtual machine is in a stable state with its current host.
The virtual machine state information (memory, registers, and network connections) is copied to the target host.
The virtual machine resumes its activities on the new host.
If errors occur during migration, the virtual machine reverts to its original state and location.