HP P6000 Continuous Access Implementation Guide (T3680-96431, August 2012)

High availability
High availability reduces the risk of downtime through redundant systems, software, and IT processes
with no SPOF. HP P6000 Continuous Access contributes to high availability by providing redundant
data. Products such as HP Metrocluster and HP Cluster Extensions provide highly available
applications. For more information about these and other high-availability products, see the HP
website:
http://docs.hp.com/en/ha.html
If your business needs high availability, but does not require disaster tolerance, local and remote
sites can be in the same room, building, or city. Distance and its effect on cost and performance
are not important issues.
Recovery time objective
The RTO is a measure of high availability—it is the length of time the business can afford to spend
returning an application to operation. It includes the time required to detect the failure, to fail over
the storage, and to restart the application on a new server. RTO is usually measured in minutes or
hours, and, occasionally, in days.
A shorter RTO increases the need for products that automatically fail over applications and data.
HP Metrocluster and HP Continentalcluster work with HP P6000 Command View to provide
application and data failover in ServiceGuard HP-UX environments. HP Cluster Extension provides
similar functionality for Microsoft Windows clusters and ServiceGuard on Linux environments. For
more information on remote replication configurations with cluster software, see “Configurations
with application failover” (page 26).
Disaster tolerance
If your business requires disaster tolerance, the location of the remote site is critical. Distance and
its relationship to cost and performance are major concerns.
Disaster tolerance and distance
Disaster tolerance uses redundant components to enable the continued operation of critical
applications during a site disaster. When two sites are separated by a distance greater than the
potential size and scope of a disaster, each site is protected from a disaster on or near the other
site. HP P6000 Continuous Access enables applications to build two copies of application data
at sites that are far enough apart to provide disaster tolerance.
Determining the minimum separation distance
In disaster-tolerant solutions, the size of the threat to each site determines the required distance
between local and remote sites. The threat radius is the distance from the center of a threat to the
outside perimeter of that threat. Figure 3 (page 16) shows sample threat radii for common threat
classifications. The required distance between the two sites is the sum of the maximum threat radius
for each site when subjected to a common threat.
Choosing the remote site 15