Need clarification between vMotion and FT

As far as I can understand in vmware the key difference between HA (High Availability) and FT (Fault Tolerance) is that is-case of failure, in HA the vm will restart so the data in memory (ram) will be lost but with FT it wont happen as its running in active active mode. So what is vMotion exactly? As far as i know it allows live mitigation, upgrade etc but if i setup HA with vMotion will it act like FT in case of disaster?

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  1. HA: “Oh crap, someone tripped over the power cord of the server that VM was running on.. Oh, look, the VM is now starting up on a new server.. Phew.. (Downtime measured in minutes, nothing saved)”

    VMotion: “Oh crap, that server the VM is running on is starting to record a BUNCH of memory errors. I should get that fixed.. I’m going to vMotion all the VM’s on it to another server and shut the server down for maintenance. Phew… Everything is still running. There might have been a minor blip in network access”

    FT: “Oh crap, that server the VM is running on is starting to record a bunch of memory errors and crashed before I could evacuate all the VM’s.. Phew.. The VM’s that were set to run FT are still up and running on another server and everything seamlessly picked up where it left off with maybe a minor blip in network access. All the VM’s that were not FT have been restarted via HA.”

  2. No, vMotion with just HA will not act like FT in an event of a disaster. In the scenario that you mentioned, vMotion is just a means of migrating a guest VM from one host to another. FT creates and maintains a second copy of a VM to be used to replace the original VM in the event of a failover situation.

  3. * HA restarts VMs on a new host when the host they were on disappears from the cluster. There is some downtime with the services on that VM as it is booted.
    * FT keeps a shadow copy of a VM running on another host that can take over without interruption if the host the primary copy is running on disappears.

    vMotion is the technology built into vmware that lets you migrate running VMs from one host to another without downtime. For example if you have host A and host B, and your VM is on host A but that host needs rebooted or needs to be shut down to install new hardware, vMotion allows you to move the VM to host B without shutting it down first.

    > if i setup HA with vMotion will it act like FT in case of disaster?

    No. vMotion is a button you can manually (or automatically) click to move VMs between hosts. All the involved hosts need to be available for you to use vMotion. HA and FT handle what to do with running VMs when the host they are running on is suddenly *unavailable.*

    It’s always best to think in terms of physical hardware when you have trouble understanding a VM concept. HA is like having a hot spare of a server. If the main server crashes, you go boot up the hot spare. You could automate this with monitoring scripts and WoL, for example. FT is more like having multiple live servers running at the same time behind a load balancer doing failover instead of balancing. vMotion is not really related to either one and there is no analog to physical processes — it’s a way to move the running processes on one physical machine to a different physical machine without interrupting them.

  4. Keep in mind that these techniques will only save you from a hardware issue.
    If the OS on the VM crashes, it will crash on both sides in FT.
    Use clustering on OS level to minimize the chance of crashing whenever possible

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