Clustering involves combining computers to create a logical computer that provides increased redundancy. A cluster uses nodes, resources, resource groups, and storage area networks. There are different types of cluster models including local quorum, single quorum device, and majority node set clusters. Storage is accessed through zoning and LUN masking to control traffic flow and provide security. Replication can be done at the block, file system, or application level between sites and can be synchronous, where an operation is complete only after replication, or asynchronous, where an operation can be considered complete before replication.

1. CLUSTERING
ERIC VANDERBURG

2. CLUSTERING
• Combining computers to create one
logical computer
• Provides increased redundancy

3. TERMS
• Node (cluster host) - A computer that is a member of a server cluster.
• Resource - entity that can be managed by a cluster and moved between
nodes.
• Resource Group - A collection of resources that can be started or stopped.
• Failover - the process of taking resources offline on one node and moving
them to another.
• Failback - returning resources back to their original node once it has
recovered.
• Quorum - the disk that holds the configuration data and all the changes that
have been made to the cluster database.
• HBA (Host Bus Adapter) - interface card that connects a cluster node to a SAN.
• Split brain - some nodes in the cluster lose contact with the quorum and one
another so each thinks that they are the only one in the cluster. This
eventually leads to data corruption.

4. INSTANCES
• Single Instance application - One application is
running on the server at a time
• Multiple instance application - multiple instances
of the same code or different pieces of code
running for the same service are running on a
cluster.
• Cloned applications - uses a relatively static dataset.
• Partitioned application - dataset is frequently updated
so the dataset is divided among partitions.

5. STORAGE
• Clusters use SANs for storage
• Cluster network should be connected directly to
the SAN and the network between them should
be isolated from the rest of the network.
• Parallel SCSI
• Fiber Channel
• If there are more than 2 nodes,m FC-SW
(Switched Fiber Channel Fabric) must be used.

6. ACCESS TO THE SAN
• Zoning - creates views called clouds that control
access and control traffic flow between the SAN.
• LUN masking - access control is provided at the
controller and port level. Policies are defined for
specific devices. Provides more granular security
than zoning.

7. CLUSTER MODELS
• Local Quorum cluster - used for testing and has only one
node.
• Single Quorum device cluster - cluster configuration data is
stored on a single device connected to all nodes in the
cluster. At least one node must be functional for the
cluster as a whole to function.
• Majority node set cluster - each node contains its own
copy of the cluster configuration data. A majority of the
nodes in the cluster must function for the cluster as a
while to function.

8. MULTISITE CLUSTER REPLICATION
• Block level replication - storage devices are
mirrored over a private network between
sites.
• File system level replication
• Application level replication

9. SYNCHRONOUS & ASYNCHRONOUS
• Synchronous replication - an operation is
not complete until it has been replicated to
partner sites
• Asynchronous replication - an operation can
be considered complete before it has been
replicated to partners.

10. SYNCHRONOUS & ASYNCHRONOUS
• Synchronous replication - an operation is
not complete until it has been replicated to
partner sites
• Asynchronous replication - an operation can
be considered complete before it has been
replicated to partners.