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warning msg--when added 1 node to cluster.pls clarify it.

    Question


  • Hi All,

    I added 1 node to my 2-nodes cluster setup and got this below warning msg:

    -------------------------------------------------------------
    The cluster has an odd number of nodes and a quorum disk.
    A quorum disk in this scenario reduces cluster high availability. Consider removing the quorum disk.
    -------------------------------------------------------------

    Does the meaning of this msg is "if cluster is having odd number of nodes then remove quorum disk to maintain HA otherwise
    if cluster is having even number of nodes then will not show above warning msg and no need to remove quorum disk ?

    Am I right ?
    Pls reply

    Thanks in advance.

    Gautam,Vikash
    Friday, May 16, 2008 11:34 AM

Answers

  • A detailed explaination of Quorum can be found http://technet2.microsoft.com/windowsserver2008/en/library/139232d7-6b02-471c-94f7-426d5fdaca711033.mspx?mfr=true

    and a webcast at http://msevents.microsoft.com/cui/WebCastEventDetails.aspx?culture=en-US&EventID=1032364842&EventCategory=4

     

    And here is my understanding on Quorum Configuration.

     

    Quorum Model has been one of the major changes in Windows Server 2008.

     

    In Windows server 2008, the High Availability of the cluster is decided on Majority of Voters. Quorum Configuration tells cluster what my voters are.

    In a Node and Disk Majority configuration, all the nodes in the cluster are voter and the disk that you select is also a voter. Where as in Node Majority, only the nodes are voter. In Node And File Share Witness, the Nodes and File share participate in voting where as in Disk Only (Legacy) quorum model, Disk is the only voter.

     

    For the cluster to remain up and running at any given point, it should have Majority of voters, in simple terms i would say (No of voters/2) + 1 would make majority.

     

    So when you add a node in a 2 Node cluster with Quorum configuration as Node and Disk Majority, you end up having 4 voters (3 Nodes and 1 Disk). Based on the above formula (4 /2 ) + 1 = 3 which means cluster can only tolerate failure of one voter (either of the nodes or the disk), so in this scenario if the quorum disk and a node or any two nodes goes down the cluster would go down. But when you change Quorum configuration to Node Majority  you end up having (3 /2) + 1 = 2. Which mean, you cluster can now tolerate failure of one node and the dependency of disk is removed.

     

    At any given point the cluster should have a quorum configuration which could tolerate max number of failures(of voters).

     

    While making a choice on Quorum Configuration you should also consider the reliability of the voters( Nodes are more reliable than disks than File Share).

     

    So the obvious better selection or choice is to switch the cluster quorum to Node Majority (having more reliable voters) and that's what Cluster Administrator suggests.

     

    A general guide line could be defined here stating that when you have even number of nodes, consider having Node and Disk Majority, if you have odd number of nodes, consider having Node Majority.

     

    Hope i have not created more confusion Smile.

     

    Thanks

    Vikas

     

    Friday, May 16, 2008 4:51 PM
  • No, an odd number of nodes is fine as long as you remember to set the quorum model correctly.  You need to evaluate what makes sense for your environment, but you may want to set up an active/active/passive environment where either active node can fail to the passive node.  Under the 2003 model, something like that could be done, but with the quorum disk as a potential single point of failure, you couldn't provide the same level of expected uptime as you now can with something like a file share witness in a three node cluster.

     

    Basically, the new quorum models give you more flexibility in building a higher available model than in the past.

     

    tgc

     

    Saturday, May 17, 2008 2:21 AM

All replies

  • A detailed explaination of Quorum can be found http://technet2.microsoft.com/windowsserver2008/en/library/139232d7-6b02-471c-94f7-426d5fdaca711033.mspx?mfr=true

    and a webcast at http://msevents.microsoft.com/cui/WebCastEventDetails.aspx?culture=en-US&EventID=1032364842&EventCategory=4

     

    And here is my understanding on Quorum Configuration.

     

    Quorum Model has been one of the major changes in Windows Server 2008.

     

    In Windows server 2008, the High Availability of the cluster is decided on Majority of Voters. Quorum Configuration tells cluster what my voters are.

    In a Node and Disk Majority configuration, all the nodes in the cluster are voter and the disk that you select is also a voter. Where as in Node Majority, only the nodes are voter. In Node And File Share Witness, the Nodes and File share participate in voting where as in Disk Only (Legacy) quorum model, Disk is the only voter.

     

    For the cluster to remain up and running at any given point, it should have Majority of voters, in simple terms i would say (No of voters/2) + 1 would make majority.

     

    So when you add a node in a 2 Node cluster with Quorum configuration as Node and Disk Majority, you end up having 4 voters (3 Nodes and 1 Disk). Based on the above formula (4 /2 ) + 1 = 3 which means cluster can only tolerate failure of one voter (either of the nodes or the disk), so in this scenario if the quorum disk and a node or any two nodes goes down the cluster would go down. But when you change Quorum configuration to Node Majority  you end up having (3 /2) + 1 = 2. Which mean, you cluster can now tolerate failure of one node and the dependency of disk is removed.

     

    At any given point the cluster should have a quorum configuration which could tolerate max number of failures(of voters).

     

    While making a choice on Quorum Configuration you should also consider the reliability of the voters( Nodes are more reliable than disks than File Share).

     

    So the obvious better selection or choice is to switch the cluster quorum to Node Majority (having more reliable voters) and that's what Cluster Administrator suggests.

     

    A general guide line could be defined here stating that when you have even number of nodes, consider having Node and Disk Majority, if you have odd number of nodes, consider having Node Majority.

     

    Hope i have not created more confusion Smile.

     

    Thanks

    Vikas

     

    Friday, May 16, 2008 4:51 PM
  • Thanks Vikas. Its really useful info for me.Now I got a feel why I have got that warning message.


    Friday, May 16, 2008 7:57 PM
  • Want to say/add one thing more.

    So that means also that its advisable to build and practice cluster of even number of nodes rather than build cluster of odd number of nodes ( in production ) ????

    Correct?


    Friday, May 16, 2008 8:02 PM
  • No, an odd number of nodes is fine as long as you remember to set the quorum model correctly.  You need to evaluate what makes sense for your environment, but you may want to set up an active/active/passive environment where either active node can fail to the passive node.  Under the 2003 model, something like that could be done, but with the quorum disk as a potential single point of failure, you couldn't provide the same level of expected uptime as you now can with something like a file share witness in a three node cluster.

     

    Basically, the new quorum models give you more flexibility in building a higher available model than in the past.

     

    tgc

     

    Saturday, May 17, 2008 2:21 AM