Fault Tolerance or we can say FT provides continuous availability for your mission critical virtual machines. So by using FT you can ensure your virtual machine will have continuous uptime and availability in case of virtual machine failure or the ESXi host failure. FT provides continuous availability for such a virtual machine by creating and maintaining another VM that is identical and continuously available to replace it in the event of a failover situation. FT works on VM level, So based on your requirements you can enable or disable FT on virtual machines.
Fault Tolerance Architecture from VMware Docs:
As you can see from above FT Architecture you can easily see How FT works.
So How Fault Tolerance works:
When you enable Fault Tolerance on the Virtual Machine, It creates secondary copy of the virtual machine. Secondary copy will be created on the other host and other shared datastore. In this VM on which you enable FT will be called as Primary and Secondary will be the replica of the Primary VM.
Once the secondary VM will be created it will be an identical copy of the Primary VM and will continuously sync data from the Primary VM so that the secondary VM can take over at any point, thereby providing Fault Tolerant protection. Here data is sync is done through FT logging Network. When the replica is created it uses storage vMotion to do the initial sync.
Primary and Secondary VM will continuously monitor each other and sync data to ensure that Fault Tolerance is maintained.
A transparent failover occurs if the host running the Primary VM fails, in which case the Secondary VM is immediately activated to replace the Primary VM. A new Secondary VM is started and Fault Tolerance redundancy is reestablished automatically. If the host running the Secondary VM fails, it is also immediately replaced. In either case, users experience no interruption in service and no loss of data.
Fault Tolerance avoids “split-brain” situations, which can lead to two active copies of a virtual machine after recovery from a failure. Atomic file locking on shared storage is used to coordinate failover so that only one side continues running as the Primary VM and a new Secondary VM is respawned automatically.
So How FT is different then HA :
HA works on the ESXi Host level, where if any of the ESXi host gets failed , HA will restart those VMs onto another ESXi Hosts. So in HA you can say your downtime is equal to restart time of Virtual Machine.
FT works on the VM level, where you configure FT on the virtual machines which are most critical and requires continuous availability. So if the VM fails the secondary copy of that VM will take over it and you can have zero downtime.
Before configuring FT there are some system requirements which needs to be fulfilled to successfully configure Fault Tolerance:
For Intel: Intel Sandy Bridge or later. Avoton is not supported
For AMD: AMD Bulldozer or later.
Network card must be at least 1Gb, with a 10Gb card recommended. A dedicated FT network is highly recommended.
You can use any of FC SAN, iSCSI or NFS storage, but not local storage. Because it requires shared storage.
FT is supported in vSphere Standard, vSphere Enterprise and vSphere Enterprise Plus Editions.
vSphere Standard and Enterprise. Allows up to 2 vCPUs
vSphere Enterprise Plus. Allows up to 8 vCPUs
Before configuring FT make sure to check for CPU and Guest OS compatibility for the Fault Tolerance.
So using Fault Tolerance you can have zero downtime and continuous availability for your virtual machines.
Check below links to configure Fault Tolerance on VM:
Information Source: VMware Docs.
Image Credit : VMware Docs.
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