Last week, I blogged about the importance of HA for unplanned host downtime. By the number of responses, this is clearly a hot topic. Today, I was going to discuss planned downtime, specifically, the differences between Quick Migration and Live Migration; however, after sifting through all that feedback last week I realized that we need to dispel some myths first…
After my last blog I received almost two dozen email telling me that VMotion was far superior for unplanned host downtime and that it was a much better HA solution because it could live migrate virtual machines. I’ve heard this fallacy espoused for many years and, folks, this simply isn’t the case.
In the case of unplanned downtime, VMotion can’t live migrate because there is no warning. Instead you must have VMware HA configured and the best it can do is restart the affected virtual machines on other nodes which is the same as what is provided with Windows Server 2008 Hyper-V and Failover Clustering.
Here are a couple of quotes from VMware’s own document, Automating High Availability (HA) Services with VMware HA.
Page 1 paragraph 2 states:
Using VMware HA, virtual machines are automatically restarted in the event of hardware failure…
Page 8 states:
How does VMware HA work?
VMware HA continuously monitors all ESX Server hosts in a cluster and detects failures. An agent placed on each host maintains a “heartbeat” with the other hosts in the cluster and loss of a heartbeat with the other hosts in the cluster and loss of a heartbeat initiates the process of restarting all affected virtual machines on other hosts.
HA monitors whether sufficient resources are available in the cluster at all times in order to be able to restart virtual machines on different physical host machines in the event of host failure.
The point being VMware HA and Hyper-V with failover clustering accomplish the same thing: virtual machines are RESTARTED on another node. No better, no worse. If you still don’t believe me, find one of your ESX Servers and go pull out the power plug. (Just don’t say I didn’t warn you.)