HP serviceguard t2808-90006 manual From both storage devices

Disaster Tolerance and Recovery in a Serviceguard Cluster

Understanding Types of Disaster Tolerant Clusters

	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.
NOTE	There are no rules 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 below.

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