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

server cluster. The correct failover and failback decisions are made

automatically, minimizing downtime and accelerating recovery.

Benefits of Cluster Extension

Cluster Extension offers a more resilient solution than Extended Distance Cluster, as it provides complete integration between

the Serviceguard application package and the data replication subsystem. The storage subsystem is queried to determine the

state of the data on the arrays.

Cluster Extension knows that application package data is replicated between two data centers. It takes advantage of this

knowledge to evaluate the status of the local and remote copies of the data, including whether the local site holds the primary

copy or the secondary copy of data, whether the local data is consistent and whether the local data is current. Depending on

the result of this evaluation, Cluster Extension decides if it is safe to start the application package, whether a

resynchronization of data is needed before the package can start or whether manual intervention is required to determine the

state of the data before the application package is started.

Cluster Extension allows for customization of the startup behavior for application packages depending on your requirements

such as data currency or application availability. This means that by default, Cluster Extension always prioritizes data

consistency and data currency over application availability. If, however, you choose to prioritize availability over currency,

you can configure Cluster Extension to start up even when the state of the data cannot be determined to be fully current,

However, the data is consistent.

Cluster Extension XP supports synchronous and asynchronous replication modes, allowing you to prioritize performance over

data currency between the data centers.

Because data replication and resynchronization are performed by the storage subsystem, Cluster Extension can provide

significantly better performance than Extended Distance Cluster during recovery. Unlike Extended Distance Cluster, Cluster

Extension does not require any additional CPU time for data replication, which minimizes the impact on the host.

There is little or no lag time writing to the replica, so the data remains current.

Data can be copied in both directions, so that if the primary site fails and the replica takes over, data can be copied back to

the primary site when it comes back up.

Disk resynchronization is independent of CPU failure. If the hosts at the primary site fail but the disk remains up, the disk

knows it does not have to be resynchronized.

Tip:

More info on HP StorageWorks XP Cluster Extension software can be

obtained from

http://www.hp.com/go/clxxp.

More info on HP StorageWorks EVA Cluster Extension software can be

obtained from

http://www.hp.com/go/clxeva.