Fault tolerance provides real-time replication of a VM on another secondary VM (which is sitting on another host) - allowing the identical secondary copy of the VM to takeover if the primary VM's host goes offline.
While this can be extremley useful in some cirucmstances there are a few drawbacks to note:
- CPU Archiecures / Vendors must be compatible with each other (i.e. they must be in the same FT processor group (http://kb.vmware.com/kb/1008027)
- If the primary VM crashes e.g. a BSOD - the other one dies too! - This is where a load balanced solution has the edge.
Before enabling fault tolerance (which I will refer to as FT from here in) on a cluster there are specific pre-requisites that must be met.
The two ESXI hosts should be part of a HA cluster - to set this up we should go to:
Inventory >> Host and Clusters >> Right-hand click on our Datacentre >> New Cluster and make sure "Turn on vSphere HA" is selected, configuring the failover settings and fo forth.
We should now drag and drop our two ESXI host's directly onto the cluster.
vSphere FT requires the following cluster requirements:
- Both ESXI hosts should have access to the same VM datastores and networks.
- Host certificate checking must be enabled within vCenter - this can performed within the vSphere client >> Administration >> vCenter Server Settings >>
- Fault Tolerance logging and VMotion networking configured: This can be performed by adding VMKernal connection via a point-to-point link between the two ESXI hosts. To set this up we go to Inventory >> Hosts and Clusters >> ESXI Host >> Conifugration >> Networking >> Hardware >> Add networking... >> VMKernel >> Create a vSphere standard switch >> Under connection types ensure that you only check either "Use this port for vMotion" or "Use this port for Fault Tolerance Logging" - as they can't reside on the same subnet! i.e. you will need to perform this process for the vMotion and Fault Tolerance Logging on each ESXI host.
Since they are both going to be p2p links we can setup the network something like the following:
FT Logging: 192.168.0.0/30
vMotion: 192.168.1.0/30
We also have specific host requirements we must meet:
- As mentioned above hosts must have processors from the FT-compatible processor group.
- Appropriate licensing must be in place to the enable FT tecnhology on the host.
Finally the VM's themsevles have requirements that must be met:
- Incompatible features / devices: Such as RDMs, Physical CD/DVD ROM Drives, USB Devices, Serial ports and so on. (See here for full list: https://pubs.vmware.com/vsphere-50/index.jsp?topic=%2Fcom.vmware.vsphere.avail.doc_50%2FGUID-05EA00B4-8142-4388-9439-F949C30ACA03.html)
- VM's must be stored on shared media such as Fibre Channel, iSCSI, NFS, or NAS.
- Virutal machines must be limited to 1 vCPU in order to work with Fault Tolerance.
Now all of the initial FT pre-requisites and setup has been performed we can FT-enable one of our VMs:
Firstly ensure that your VM is turned off! Now from the vSphere web interface simply right-hand click on the relevent machine >> Fault Tolerance >> and hit Enable. We will be prompted to indicate where we wish to store the seconday copy of our VM (which will be on shared storage on another device!)
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