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Understanding VMware Fault Tolerance
Purpose
VMware Fault Tolerance (FT) provides continuous availability for applications by creating a live shadow instance of a virtual machine that is in virtual lockstep 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.
Resolution
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 the 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 a FT environment, a 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 the primary fails, the secondary virtual machine takes over with no interruptions to 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/backup hosts are down. Backup goes live and becomes new primary if it declares the current primary dead.
A method to distinguish a failed host from a network failure is needed to prevent split-brain syndrome. FT uses an atomic operation on the shared VMFS to accomplish this.
VMware FT Migration Transition States
VMware FT tracks failover operations with a variety of states. Additional information regarding these transition states can be found at VMware Fault tolerance migration transition states (1010634).
Enabling VMware FT
VMware FT is enabled within the VMware Infrastructure Client by right clicking on a virtual machine and selecting Enable VMware FT. For this option to be available, the cluster in which the virtual machine resides must meet several requirements. Additional information regarding these requirements can be found in The turn on Fault Tolerance option is disabled (1010631).
For more information about VMware Fault Tolerance, refer to the article: VMware Fault Tolerance FAQ (1013428)
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- KB Article: 1010601
- Updated: Aug 20, 2009
- Products:
VMware ESX
VMware ESXi - Product Versions:
VMware ESX 4.0.x
VMware ESXi 4.0.x Embedded
VMware ESXi 4.0.x Installable