VMware Fault Tolerance keeps two virtual machines running in parallel to provide application high-availability.  If the primary VM fails the secondary copy can seamlessly continue processing the workload. The workload transfer from primary to secondary occurs transparently with no impact to end-users.

VMware FT does this by capturing all inputs to the primary VM and forwarding them to the secondary VM. One issue to watch out for is that disk read-intensive workloads have the ability to generate massive amounts of network traffic between the ESX hosts running the primary and secondary copies of an FT pair.  For example, a virtual machine doing disk reads at a 80MB/sec generates at least 80 x 8 = 640Mb/sec of logging traffic.

 To reduce the logging traffic on the network it's possible to configure the secondary VM to perform disk reads directly from the shared disk, eliminating the need to copy this data over the network.  VMware has documented how to do this in the KB artical <a href="http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&amp;cmd=displayKC&amp;externalId=1011965">here</a>.