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

9
Benefits of Extended Distance Cluster
The following table discusses the benefits of Extended Distance Cluster.
Benefits of Extended Distance Cluster
This configuration implements a single Serviceguard cluster across two data centers, and uses Multiple Device (MD) driver for
data replication.
You can choose any mix of Fibre Channel-based storage supported by Serviceguard that also supports the QLogic driver
multipath feature.
This configuration might be the easiest to understand because it is similar in many ways to a standard Serviceguard cluster.
Application failover is minimized. All disks are available to all nodes, so that if a primary disk fails but the node stays up and
the replica is available, there is no failover. (The application continues to run on the same node while accessing the replica.)
Data copies are peers, so there is no issue with reconfiguring a replica to function as a primary disk after failover.
Writes are synchronous, so data remains current between the primary disk and its replica, unless the link or disk is down.
Tip:
More info on HP Serviceguard Extended Distance Cluster for Linux can be
obtained from
http://www.hp.com/go/xdclinux.
HP StorageWorks Cluster Extension Software
Cluster Extension for Linux is similar to an HP-UX metropolitan cluster and is a cluster that has alternate
nodes located in different parts of a city or in nearby cities. Locating nodes further apart increases the
likelihood that alternate nodes are 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 a third
location for arbitrator nodes or a quorum server. Additionally, as with an extended distance cluster,
the distance separating the nodes in a metropolitan cluster is limited by the data replication and
network technology available.
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.
On Linux, the metropolitan cluster is implemented using Cluster Extension:
• Cluster Extension for XP
• Cluster Extension for EVA
Figure 5 shows the Serviceguard for Linux and Cluster Extension architecture and product suite that
were specifically used for the Disaster Proof video.