Thursday, 15 November 2012

Windows Clustering in Virtual Environments

Clustering in virtual environments is often something that is misunderstood, virtualisation provides many high-availability benefits out of the box but falls short under some conditions.  When you are next considering high-availability in a virtual environment ask yourself the following:-

  • What happens if I have an OS failure?
  • What happens if I want to do an online upgrade/patch?
  • In the case of SQL 2012, how will you auto failover your availability group?
These kind of things can be often overlooked in an age where the OS is more stable and the NT 4 failures of old become a thing of the past.  SQL 2012 does not support auto failover without a quorum and this will give you a headache in a virtual environment.

Ha, ha I hear you say, I have snapshots!!! A snap-shot will not help you reduce downtime.  Restoring or rolling back a snapshot takes time, this is critical time for 24/7/365 businesses and sometimes is not an option.  Snapshots also have an overhead in the differing management.  When relying on snapshots, in an upgrade scenario, you will need downtime whilst applying that critical update if you do not have a secondary node.  This might not make users happy.

These kinds of questions are still difficult to resolve and are actually harder today than in the bare metal days of the past.  The support of MSCS in a virtual environment is a quagmire of support matricies and conditions.  You will need to ensure that the Hypervisor is limiting its resource interaction and that iSCSI is your quorum disk.  There are other options but this is the most common scenario.  Unless you are on Hyper-V of course..

So don't avoid clusters in a virtual environment, understand the business requirement and leave nothing to chance!  Clustering still has its place today for obvious reasons, don't be told otherwise.

If you are planning clustering on vSphere and rely on MS support, please fully understand that Microsoft support are fussy.  You'll need to have followed the following to the letter of the law.

http://www.vmware.com/pdf/vsphere4/r40/vsp_40_mscs.pdf
http://kb.vmware.com/kb/1004617






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