Types of Disaster Tolerant Clusters

To protect against multiple points of failure, cluster components must be geographically dispersed: nodes can be put in different rooms, on different floors of a building, or even in separate buildings or separate cities. The distance between the nodes is dependent on the types of disaster from which you need protection, and on the technology used to replicate data. Three types of disaster-tolerant clusters are described in this guide:

Extended Distance Clusters
Metropolitan Clusters
Continental Clusters

These types differ from a simple local cluster in many ways. Extended distance clusters and metropolitan clusters often require right-of-way from local governments or utilities to lay network and data replication cable. This can complicate the design and implementation. They also require a different kind of control mechanism for ensuring that data integrity issues do not arise. Typically, metropolitan clusters use an arbitrator site containing additional cluster nodes instead of the cluster lock disk. Continental clusters span great distances and operate by replicating data between two completely separate local clusters.

Extended Distance Clusters

An extended distance cluster (also known as extended campus cluster) has alternate nodes located in different data centers separated by some distance. Extended distance clusters are connected using a high speed cable that guarantees network access between the nodes as long as all guidelines for disaster tolerant architecture are followed. Extended distance clusters were formerly known as campus clusters, but that term is not always appropriate because the supported distances have increased beyond the typical size of a single corporate campus. The maximum distance between nodes in an extended distance cluster is set by the limits of the data replication technology and networking limits. An extended distance cluster is shown in Figure 1-3 “Extended Distance Cluster ”.

Figure 1-3 Extended Distance Cluster

Architecture requirements for several types of extended distance clusters are described more fully in Chapter 2. Extended distance clusters can be configured over shorter distances using FibreChannel mass storage, or over distances as great as 100 km using storage and networking routed over links extended via DWDM.

Metropolitan Cluster

A metropolitan cluster is a cluster that has alternate nodes located in different parts of a city or in adjacent cities. Putting nodes further apart increases the likelihood that alternate nodes will be available for failover in the event of a disaster. The architectural requirements are the same as for an extended distance cluster, with the additional constraint of the third location. And as with a campus cluster, the distance separating the nodes in a metropolitan cluster is limited by the data replication and network technology available.




	NOTE: While it is possible to configure physical data replication through products such as HP’s XP Series disk arrays with Continuous Access XP or Symmetrix EMC SRDF, it is still necessary to provide for high availability at the local level through RAID or mirroring.

In addition, there is no hard requirement on how far the third location has to be from the two main data centers. The third location can be as close as the room next door with its own power source or can be as far as in a site across town. The distance between all three locations dictates the level of disaster tolerance a metropolitan cluster can provide.

Metropolitan cluster architecture is implemented through two HP products:

Building Disaster-Tolerant Serviceguard Solutions with Continuous Access XP (described in Chapter 4)
Building Disaster Tolerant Serviceguard Solutions with EMC SRDF (described in Chapter 6)

Metropolitan cluster architecture is shown in Figure 1-4 “Metropolitan Cluster ”.

Figure 1-4 Metropolitan Cluster

A key difference between extended distance clusters and metropolitan clusters is the data replication technology used. The extended distance cluster uses FibreChannel and HP-UX supported software mirroring for data replication. Metropolitan clusters provide extremely robust hardward-based data replication available with specific disk arrays based on the capabilities of the HP StorageWorks E Disk Array XP series or the EMC Symmetrix array.

Continental Cluster

A continental cluster provides an alternative disaster tolerant solution in which distinct clusters can be separated by large distances, with wide area networking used between them. Continental cluster architecture is implemented via the ContinentalClusters product, described fully in Chapter 4. The design is implemented with two distinct Serviceguard clusters that can be located in different geographic areas. In this architecture, each cluster maintains its own quorum, so an arbitrator data center is not used for a continental cluster. A continental cluster can use any WAN connection via a TCP/IP protocol; however, due to data replication needs, high speed connections such as T1 or T3/E3 leased lines or switched lines may be required. See Figure 1-5 “Continental Cluster ”.




	NOTE: A continental cluster can also be built using two clusters that communicate over shorter distances using a conventional LAN.

Figure 1-5 Continental Cluster

Continentalcluster provides the flexibility to work with any data replication mechanism and it provides pre-integrated solutions that uses HP StorageWorks Continuous Access XP or EMC Symmetrix Remote Data Facility for data replication via Metrocluster products.

The points to consider when configuring a continental cluster over a WAN are:

Inter-cluster connections are TCP/IP based.
The physical connection is one or more leased lines managed by a common carrier. Common carriers cannot guarantee the same reliability that a dedicated physical cable can. The distance can introduce a time lag for data replication, which creates an issue with data currency. This could increase the cost by requiring higher speed WAN connections to improve data replication performance and reduce latency.
Operational issues, such as working with different staff with different processes, and conducting failover rehearsals, are made more difficult the further apart the nodes in the cluster are.

Continental Cluster With Cascading Failover

Another way of setting up your continental cluster is the use of cascading failover, an option available for users of the EMC Symmetrix or HP StorageWorks disk arrays. Cascading failover means that applications are configured to fail over on nodes within two data centers in either cluster. They fail over to an alternate cluster if the entire cluster is down. Data replication also follows the cascading model. Data is replicated from the primary disk array to the secondary disk array in the Metrocluster, then data is replicated to the third disk array in the Serviceguard recovery cluster.

A continental cluster with cascading failover uses three main data centers distributed between a metropolitan cluster, which serves as a primary cluster, and a standard cluster, which serves as a recovery cluster.

Cascading Failover Using Continuous Access XP

The configuration uses three XP HP StorageWorks disk arrays, two of which are part of the metropolitan cluster and the other attached to the recovery cluster. The data centers are distributed as follows:

Primary XP disk array—on the site that holds the primary copy of the data, located in the primary cluster.
Secondary XP disk array—on the site that holds a remote mirror copy of the data, located in the primary cluster.
Arbitrator—a third location that contains the arbitrator nodes, located in the primary cluster.
Recovery XP disk array—on a site that holds a remote mirror copy of the data, located in the recovery cluster.

Figure 1-6 “Cascading Failover Data Center Distribution Using XP CA” illustrates data centers, clusters, nodes and Continuous Access XP in a cascading failover configuration. Contact your HP Representative for details on setting up data replication for this type of cluster.

Figure 1-6 Cascading Failover Data Center Distribution Using XP CA

Cascading Failover Using EMC SRDF

The configuration uses three EMC Symmetrix frames, two of which are part of the metropolitan cluster and the other attached to the recovery cluster. The data centers are distributed as follows:

Primary Symmetrix—on the site that holds the primary copy of the data, located in the primary cluster.
Secondary Symmetrix—on the site that holds a remote mirror copy of the data, located in the primary cluster.
Arbitrator—a third location that contains the arbitrator nodes, located in the primary cluster.
Recovery Symmetrix—on a site that holds a remote mirror copy of the data, located in the recovery cluster.

Figure 1-7 “Cascading Failover Data Center Distribution Using EMC SRDF” illustrates data centers, clusters, nodes and Symmetrix frames in a cascading failover configuration. Refer to Chapter 7 for details on setting up data replication for this type of cluster.

Figure 1-7 Cascading Failover Data Center Distribution Using EMC SRDF

Types of Disaster Tolerant Clusters

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