>What is Clustering. Briefly define & explain it ?
Clustering is a technology, which is used to provide High Availability for mission critical applications. We can configure cluster by installing MCS (Microsoft cluster service) component from Add remove programs, which can only available in Enterprise Edition and Data center edition.
>Types of Clusters ?
In Windows we can configure two types of clusters
1. NLB (network load balancing) cluster for balancing load between servers. This cluster will not provide any high availability. Usually preferable at edge servers like web or proxy.
2. Server Cluster: This provides High availability by configuring active-active or active-passive cluster. In 2 node active-passive cluster one node will be active and one node will be stand by. When active server fails the application will FAILOVER to stand by server automatically. When the original server backs we need to FAILBACK the application
> What is Quorum ? A shared storage need to provide for all servers which keeps information about clustered application and session state and is useful in FAILOVER situation. This is very important if Quorum disk fails entire cluster will fails.
>Why Quorum is necessary ?
When network problems occur, they can interfere with communication between cluster nodes. A small set of nodes might be able to communicate together across a functioning part of a network, but might not be able to communicate with a different set of nodes in another part of the network. This can cause serious issues. In this “split” situation, at least one of the sets of nodes must stop running as a cluster.
To prevent the issues that are caused by a split in the cluster, the cluster software requires that any set of nodes running as a cluster must use a voting algorithm to determine whether, at a given time, that set has quorum. Because a given cluster has a specific set of nodes and a specific quorum configuration, the cluster will know how many “votes” constitutes a majority (that is, a quorum). If the number drops below the majority, the cluster stops running. Nodes will still listen for the presence of other nodes, in case another node appears again on the network, but the nodes will not begin to function as a cluster until the quorum exists again.
For example, in a five node cluster that is using a node majority, consider what happens if nodes 1, 2, and 3 can communicate with each other but not with nodes 4 and 5. Nodes 1, 2, and 3 constitute a majority, and they continue running as a cluster. Nodes 4 and 5 are a minority and stop running as a cluster, which prevents the problems of a “split” situation. If node 3 loses communication with other nodes, all nodes stop running as a cluster. However, all functioning nodes will continue to listen for communication, so that when the network begins working again, the cluster can form and begin to run.
> Different types of Quorum in Windows server 2008 ?
1.Node Majority - Used when Odd number of nodes are in cluster.
2.Node and Disk Majority - Even number of nodes(but not a multi-site cluster)
3.Node and File Share Majority - Even number of nodes, multi-site cluster
4.Node and File Share Majority - Even number of nodes, no shared storage
> Different types of Quorum in Windows server 2003 ? Standard Quorum : As mentioned above, a quorum is simply a configuration database for MSCS, and is stored in the quorum log file. A standard quorum uses a quorum log file that is located on a disk hosted on a shared storage interconnect that is accessible by all members of the cluster.
Standard quorums are available in Windows NT 4.0 Enterprise Edition, Windows 2000 Advanced Server, Windows 2000 Datacenter Server, Windows Server 2003 Enterprise Edition and Windows Server 2003 Datacenter Edition.
Majority Node Set Quorums : A majority node set (MNS) quorum is a single quorum resource from a server cluster perspective. However, the data is actually stored by default on the system disk of each member of the cluster. The MNS resource takes care to ensure that the cluster configuration data stored on the MNS is kept consistent across the different disks.
Majority node set quorums are available in Windows Server 2003 Enterprise Edition, and Windows Server 2003 Datacenter Edition.
>Explain about each Quorum type ?
Node Majority: Each node that is available and in communication can vote. The cluster functions only with a majority of the votes, that is, more than half.
Node and Disk Majority: Each node plus a designated disk in the cluster storage (the “disk witness”) can vote, whenever they are available and in communication. The cluster functions only with a majority of the votes, that is, more than half.
Node and File Share Majority: Each node plus a designated file share created by the administrator (the “file share witness”) can vote, whenever they are available and in communication. The cluster functions only with a majority of the votes, that is, more than half.
No Majority: Disk Only: The cluster has quorum if one node is available and in communication with a specific disk in the cluster storage.