Two Data Center Architecture

The two data center architecture is based on a standard Serviceguard configuration with half of the nodes in one data center, and the other half in another data center. Nodes can be located in separate data centers in the same building, or even separate buildings within the limits of Fibre Channel technology. Configurations with two data centers have the following requirements:

In order to maintain cluster quorum after the loss of an entire data center, you must configure dual cluster lock disks (one in each data center). Since cluster lock disks are only supported for up to four nodes, the cluster can contain only two or four nodes. Serviceguard does not support dual lock LUNs, so lock LUNs cannot be used in this configuration. When using dual cluster lock disks, there exists a chance of Split Brain Syndrome (where the nodes in each data center form two separate clusters, each with exactly one half of the cluster nodes) if all communication between the two data centers is lost and all nodes remain running. The Serviceguard Quorum Server prevents the possibility of split brain; however the Quorum Server must reside in a third site. Therefore a three data center cluster is a preferable solution, to prevent split brain, and the only solution if dual cluster lock disks cannot be used, or if the cluster must have more than four nodes.
Two data center configurations are not supported if SONET is used for the cluster interconnects between the Primary data centers.
There must be an equal number of nodes (one or two) in each data center.
To protect against the possibility of a split cluster inherent when using dual cluster lock, at least two (three preferred) independent paths between the two data centers must be used for heartbeat and cluster lock I/O. Specifically, the path from the first data center to the cluster lock at the second data center must be different than the path from the second data center to the cluster lock at the first data center. Preferably, at least one of the paths for heartbeat traffic should be different from each of the paths for cluster lock I/O.
No routing is allowed for the networks between the data centers, except in Cross-Subnet configurations.
MirrorDisk/UX mirroring for LVM and VxVM mirroring are supported for clusters of two or four nodes. However, the dual cluster lock devices can only be configured in LVM Volume Groups.
CVM 3.5, 4.1 or 5.0 mirroring is supported for Serviceguard and EC RAC clusters using CVM or CFS. However, the dual cluster lock devices must still be configured in LVM Volume Groups. Since cluster lock disks are only supported for up to four nodes, the cluster can contain only two or four nodes.
MirrorDisk/UX mirroring for Shared LVM volume groups is supported for EC RAC clusters containing two nodes.




	NOTE: For the most up-to-date support and compatibility information see the SGeRAC for SLVM, CVM & CFS Matrix and Serviceguard Compatibility and Feature Matrix on `http://docs.hp.com`-> High Availability -> Serviceguard Extension for Real Application Cluster (ServiceGuard OPS Edition) -> Support Matrixes

Advantages and Disadvantages of a Two Data Center Architecture

The advantages of a two data center architecture are:

Lower cost.
Only two data centers are needed, meaning less space and less coordination between operations staff.
No arbitrator nodes are needed.
All systems are connected to both copies of data, so that if a primary disk fails but the primary system stays up, there is a greater availability because there is no package failover.

The disadvantages of a two data center architecture are:

There is a slight chance of split brain syndrome. Since there are two cluster lock disks, a split brain syndrome would occur if the following happened simultaneously:
- All heartbeat networks fail.
- All disk links fail. The disk link from data center A to cluster lock disk B fails and the disk link from data center B to cluster lock disk A fails.
The chances are slight, however these events happening at the same time would result in split brain syndrome and probable data inconsistency. Planning different physical routes for both network and data connections or adequately protecting the physical routes greatly reduces the possibility of split brain syndrome.
Software mirroring increases CPU overhead.
The cluster must be either two or four nodes with cluster lock disks. Larger clusters are not supported due to cluster lock requirements.
Although it is a low cost solution, it does require some additional cost:
- Fibre Channel links are required for both local and remote connectivity.
- All systems must be connected to multiple copies of the data and to both cluster lock disks.

Two Data Center Architecture

Technical documentation

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Advantages and Disadvantages of a Two Data Center Architecture