Understanding VMware Fault Tolerance

VMware Fault Tolerance (FT) provides continuous availability for applications by creating a live shadow instance of a virtual machine that is in virtual lock-step with the primary instance. By allowing instantaneous failover between the two instances in the event of hardware failure, VMware Fault Tolerance eliminates even the smallest of data loss or disruption.


VMware Fault Tolerance (FT) works by creating an identical copy of a virtual machine. One copy of the virtual machine, called the primary, is in an active state, receiving requests, serving information and running applications. Another copy, called the secondary, receives the same input that is received by the primary.

In an FT environment, one virtual machine runs as a primary and FT runs a secondary virtual machine on a different ESX host. The secondary virtual machine shares the primary's virtual disks. The virtual machines are kept in lock-step via logging information sent over a private network connection. The primary is the sender of this logging information and the secondary only listens. FT is based on VMware Record/Replay technology.

In the event that the primary's ESX host fails, the secondary virtual machine takes over without interrupting applications.

VMware FT provides more continuity than VMware HA because FT does not require a virtual machine restart and the secondary virtual machine immediately comes online with all, or almost all state information preserved.

Virtual machines protected by FT are not handled by VMware HA for restart priority. It is considered disabled in the restart priority.

Determining Node Failure
VMware FT uses network heartbeats to determine when primary and backup hosts are down. Backup goes live and becomes the new primary if it declares the current primary dead.
FT uses an atomic operation on the shared VMFS to distinguish between a failed host from a network failure.

VMware FT Migration Transition States
VMware FT tracks failover operations with a variety of states. For more information on transition states, see - VMware Fault tolerance migration transition states (1010634).

ESX 4.0 and ESXi 4.0 shutdown and reboot commands

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When virtual machines are running, ESX might not clear the RAID controller's cache if you shut down or reboot ESX host by using the following commands on the service console:

• reboot -f 
• halt
• shutdown

Solution
You can shut down or reboot ESX 4.0 or ESXi 4.0 hosts using any of the following methods:


ESX 4.0
Log in to the ESX service console and perform one of the following steps from the service console to shutdown or reboot ESX 4.0 hosts.


Run the shutdown –r now command to reboot the system.
Note: This command shuts down the virtual machines running on the ESX 4.0 hosts.
Run the reboot command to reboot the system.
Run the poweroff command to shut down ESX. After the shutdown, a message indicates that it is safe to power off your system. Press the power button until the machine powers off. You can then manually reboot the system.


ESXi 4.0
In the console screen of the ESXi 4.0 host, press Ctrl+Alt+F2 to see the Direct Console User Interface (DCUI) screen.
In the DCUI screen, press F12 to view the shutdown related options for the ESXi host.
Press F2 to shut down.
Press F11 to reboot.


ESX 4.0 or ESXi 4.0
From vSphere Client
Before shutting down or rebooting the ESX 4.0 or the ESXi 4.0 hosts, ensure that the hosts are put in maintenance mode. Powering off a managed host disconnects it from vCenter Server, but does not remove it from the inventory.
Shut down or VMotion all virtual machines running on ESX 4.0 or ESXi 4.0 hosts.
Put the ESX 4.0 or ESXi 4.0 hosts in the maintenance mode.
Select the ESX 4.0 or the ESXi 4.0 host you want to shut down.
Right-click the ESX 4.0 or the ESXi 4.0 host that you want to shut down, and select Reboot or Shut Down.
If you select Reboot, the ESX 4.0 or the ESXi 4.0 host shuts down and reboots.
If you select Shut Down, the ESX 4.0 or the ESXi 4.0 host shuts down. You must manually power the system back.
Provide a reason for the shut down or reboot. This information is added to the log.


From vCLI or vMA
To put ESX 4.0 or ESXi 4.0 hosts in the maintenance mode, run the following command from vMA (vSphere Management Assistant) or vCLI (vSphere Command-Line Interface) console screen:

/usr/lib/vmware-vicli/apps/host/hostops.pl --target_host --operation enter_maintenance --url https:///sdk/vimService.wsdl 

To reboot ESX 4.0 or ESXi 4.0 hosts, run the following command from vMA or vCLI console screen:

/usr/lib/vmware-vicli/apps/host/hostops.pl --target_host --operation reboot --url https:///sdk/vimService.wsdl 

To shut down ESX 4.0 or ESXi 4.0 hosts, run the following command from vMA or vCLI console screen:

/usr/lib/vmware-vicli/apps/host/hostops.pl --target_host --operation shutdown --url https:///sdk/vimService.wsdl