VMware Fault Tolerance

What is Fault Tolerance ?

  • FT is a technology used by VMware to provide zero downtime for a virtual machine
  • Continuous protection for a VM even in case of a host failure
  • FT is enabled only on a VM basis
  • When enabled, FT creates a secondary VM in another ESXi
  • Uses Record and Replay technology & vLockstep technology

Record and Replay technology

  • This technology records all events in the primary VM and replays in the secondary VM( Even a mouse click) but only the primary VM produces output such as disk writes and network transmits
  • It is done using a FT logging NIC
  • The output of the secondary VM is suppressed by the ESXi and is not in the network until the primary fails
  • If the primary fails, the secondary becomes the new primary

vLockstep technology

  • This technology uses a second identical processor to monitor and verify the operations of the first processor
  • Both processors receive the same inputs so the operation state of both processors will be identical
  • Fault Tolerance feature relies on vLockstep technology to sync the CPU operations of a VM between two hosts so they are in identical states


  • FT requires newer processor models from AMD and Intel which supports vLockstep technology
  • All hosts must be running the same build of VMware ESX
  • Shared storage is required as a single vmdk is being used by two VMs in two separate hosts
  • HA must be enabled
  • VMs must be single-processor (till ESXi 5.x...From 6.0 this has been changed to 4 processors)
  • All VM disks must be “thick”
  • No devices like USB, physical CD-ROM, floppy, physical RDMs are permitted
  • Snapshots must be removed before enabling FT
  • Taking snapshot of a VM in FT is not possible
  • Storage vMotion operation is not permitted for a VM in FT
  • FT enabled virtual machine is automatically configured as DRS-disabled but initially the secondary VM is placed by DRS when FT is enabled


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