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What is High Availability(HA)? What is Failover? The Basics.

What is High Availability(HA)? What is Failover? The Basics.



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  1. High availability (HA) is the ability of a VM, in a cluster, to recover, either passively or proactively, in the event of host failure. Passive HA is when a VM sitting on a host, that catastrophically fails, is automatically restarted on another host in the same cluster. There is a brief outage of the VM while it boots, but, being a VM, it should be very quick. Proactive HA is a recent feature added in 6.7. Hosts are monitored for conditions that signal an imminent failure, and the host is put into maintenance mode, causing all its VMs to vMotion to other hosts, sparing any guests from having to reboot at all.

    The only downside of passive HA is that VMs on the host are ungracefully powered off. This causes any in-flight transactions to be lost. For stateless apps, like web servers, this isn’t really a problem as the user can simply resend the request once the VM boots on another host. For VMs involved in critical activities like processing financial transactions or real-time command and control, losing in-flight transactions is not desirable. That is where Fault Tolerance (FT) comes in.

    FT actually creates two VMs instead of one. The second VM always runs on a different host, and it behaves like a shadow of the primary VM. Every transaction that is processed by the primary is also seen by the shadow. The two servers are clones acting as one, except the shadow sends no output. Only when the cluster detects the primary has failed, does the shadow become real, and starts responding immediately as if it were the primary all along.

  2. I’ve always found the name “HA” to be misleading. Don’t get me wrong, it works great, but it’s really “Quick Recovery” not “High Availability.”

  3. I did a shot every time the word “availability” was said… after a bit you should have just said HA lol I know for a fact I cannot say “high availability” 10 times fast without twisting those words 😉

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