Building a Disaster-proof Data Center with HP Serviceguard for Linux, June 2007

Extended distance clusters

Note:

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.

An extended distance cluster (also known as an extended campus cluster) is a normal Serviceguard

cluster that has alternate nodes located in different data centers, separated by some distance with a

third location supporting the quorum service. 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. 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 3.

Note:

There are no guidelines or recommendations on how far the third location

must 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 among all three locations dictates the level of

disaster tolerance an extended distance cluster can provide.

In an extended distance cluster for data replication the Multiple Disk (MD) driver is used. Using the

MD kernel driver, you can configure RAID 1 (mirroring) in your cluster. In a dual data center setup, to

configure RAID 1, one LUN from a storage device in data center 1 is coupled with a LUN from a

storage device in data center 2. As a result, the data that is written to this MD device is

simultaneously written to both devices. A package that is running on one node in one data center has

access data from both storage devices.

The two recommended configurations for the extended distance cluster are both described in Figure