Hyperv Cluster - how does it work with node failure

Posted on 2011-05-02
Medium Priority
Last Modified: 2012-06-27
Hi we have 3 hyper-v nodes (enteprise core) all connected to a SAN and using CSV for shared area for virtual machine config and hard disks.  We have tested Live Migration and it moves from one node to another (taking approx 30 seconds).  However LM aside in a non controlled way how should the failover work?  
My understanding of LM is that the physical running RAM is synced to the other node so that it can take other however in the event of a physical failure this could not happen and the server taking over the VM would not have any RAM contents.
How should/does this work please?
Question by:nmxsupport
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LVL 124
ID: 35504271
The Guest Virtual Machines would be-restarted on the other Node, so there would be downtime of a few minutes waiting for the Guest VM to restart.

Planned Versus Unplanned Failover Process

In a planned failover scenario, whether based on a need to perform maintenance on a Hyper-V host or to rebalance the load across Hyper-V hosts through quick migration of a virtual machine, the migration process can occur without data loss and with minimal service interruption. To accomplish this, a virtual machine is placed in saved state, which results in the active memory and processor state being captured to disk, and processing is suspended. Essentially, the storage resources ownership is then transferred to the target cluster node, the active memory and processor state is loaded, and processing is resumed. Depending on the underlying storage and the size of the state data that is reloaded, the entire process can take place in a few seconds.

In an unplanned failover scenario, caused by a hardware problem or other unforeseen issue, the Hyper-V host crashes along with all the virtual machines. Because the virtual machines crash before state can be saved, the migration process loses data in active memory. However, because the Hyper-V host is part of a failover cluster, the storage resources ownership will be transferred to another cluster node and virtual machines restarted on that Hyper-V host.

See this article


Author Comment

ID: 35504331
Thanks for the link.  To confirm in an unplanned scenario, the VM will have to re-start on another node in a crashed state, similar to turning the server on and off?
LVL 124

Accepted Solution

Andrew Hancock (VMware vExpert / EE MVE^2) earned 2000 total points
ID: 35504353
That is correct. the Cluster doesn't know, the physical host is about to go down, so cannot prepare for it. Unlike, when you schedule a Planned Move, where it saves the state of the guest virtual machine.

So you are correct, it would restart after a crashed state, just like switching a server on and off.

But as it's Virtual, it will recover, and start-up much faster. So typical outage times 30secs - 2mins.


Author Comment

ID: 35504403
Okay thanks again for the confirmation, at least that has put it straght in my head.  I therefore need to look at application level clustering rather than HV clustering alone!
LVL 124
ID: 35504513
Correct if you are after serious High Availability.

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