Need to understand HA and FT option in VMware ESXi 5.0.0

I have a Data center created in the vSphere client with 3 ESX host that are clustered. We have vSphere 5 Enterprise Plus license edition. All 3 ESX host are put in a cluster under a datacenter. They are all powered on.  As it currently stands, HA, DRS and FT are not enabled. To enable both these options, do the ESX host need to be shut off? Or do we have to create a new cluster and redo all this again just to enable all those features.

Can you also give a brief explanation of what each one of those feature does (FT, DRS, and HA), and why we would want to enable them?
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coolsport00Connect With a Mentor Commented:
1. Do the Hosts or VMs need to be powered off? Absolutely not. You're golden. Just enable the settings and agents, etc. will be deployed to the Hosts in the Cluster as needed.

2. What each feature is:
    a. HA - High Availability; what this does is enables VMs to continue to run, with very minimal downtime, in the event of an ESXi Host failure. When enabled, vCenter deploys a HA agent to each Host in the Cluster. After the agent is deployed, vCenter is pretty much taken out of the HA functionality and Host 'election' (which is a Master & which is a Slave) is done by some algorithm by Hosts in the Cluster. Heartbeats are disseminated between each of the Hosts. If after a certain interval, a heartbeat for a specific Host isn't detected, it will be deemed "failed". VMs will be restarted, if need be, on remaining operational Hosts in the Cluster. The VM downtime that happens is the amt of time it takes for the VM to restart.
    b. DRS - Distributed Resource Scheduler; what this does is enable resource (CPU, Memory) load balance between all ESXi Hosts in the Cluster. If a Host is getting its resources consumed more than other Hosts in the Cluster, DRS 'sees' this and, again based on a certain algorithm, will VMotion VMs off the Host that has high resource consumption to another Host with resources that are not as consumed.
    c. FT - Fault Tolerance; what this does is create a duplicate VM of the source VM you enable FT for. The duplicate gets placed on another ESXi Host and synchronous replication of the VM state occurs between the source VM and this FT (duplicate) VM, for no downtime of the VM in the event of an ESXi Host failure. NOTE: this is not the same as Microsoft Clustering. So, if the source VM blue screens, the FT/duplicate VM will blue screen as well.

Why would you enable any of the above features? Well, I kind of explained the reason in the descriptions. You enable HA for minimal downtime of your VMs (servers). Yes, there is still some downtime, but typically only about much time as it takes for a VM to restart. FT is for 0 downtime of a critical VM that needs to stay up in the event of a Host failure. DRS is a nice feature to enable so no one ESXi Host in a Cluster gets its resources consumed more than the others. Typically, you should, when creating new VMs & adding them to the Cluster, allocate that VM to a Host with resources not used as much as other Hosts. If you enable DRS to do so, it can place the VM on the best suited Host for you.

What determines whether you enable, 1, 2, or all 3 of those features is your DR policy and/or RTO & RPO policies for downed systems. If you have SLAs to maintain, that will dictate what you 'should' enable, and how to configure them.

Hope that helps.

The key difference between VMware's Fault Tolerance (FT) and High Availability (HA) products is interruption to virtual machine (VM) operation in the event of an ESX/ESXi host failure. Fault-tolerant systems instantly transition to a new host, whereas high-availability systems will see the VMs fail with the host before restarting on another host.

Please refer below link for more information on FT and HA.
Andrew Hancock (VMware vExpert / EE MVE^2)Connect With a Mentor VMware and Virtualization ConsultantCommented:
You will need to create a Cluster.

Do you have a SAN or Shared Storage?

See this document briefs

VMware vSphere Availability Features

VMware vMotion Product Brief

VMware High Availability - Decrease Downtime and Reduce Risk

VMware High Availability (HA) Product Briefs

VMware Distributed Resource Scheduler (DRS) - Dynamic Resource Balancing

VMware Distributed Resource Scheduler (DRS) Product Briefs

VMware Fault Tolerance - Deliver 24 X 7 Availability

VMware Fault Tolerance Product Brief
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jaedenoneAuthor Commented:
coolsport00 and hanccocka,

I'm getting mixed answers here. So can i simply enable FT, or HA while all the ESX are powered on? I do not see the option to do so when I right click and looked in properties. That is why I am assuming the host must be shut down first. They are already part of a cluster.
Andrew Hancock (VMware vExpert / EE MVE^2)VMware and Virtualization ConsultantCommented:
If you want to enable VMware HA, Edit the Cluster Settings, and select VMware HA.

This can be enabled whilst VMs are Power-ON.

Make sure first, you have configured your Hosts correctly for HA.

as for VMware FT, this feature is enabled per Virtual Machine. It's a little more involved

see here¿
Yes, you can enable HA with Hosts/VMs on...they have to be on for the HA agent (FDM agent) to be deployed to the Hosts. Rt-click the Cluster > Edit Settings, select vSphere HA and click the box to enable it. Then modify the HA settings as needed.

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