Introduction to Virtualization - Oracle Solaris 10 System Virtualization Essentials - page 17

any one computer or VE does not cause a service outage. The remaining entities
continue providing the service, but the load-balancing mechanism no longer sends
transactions to the failed unit.
Because no service outage occurs, this model is often preferred to the simpler
failover model. Nevertheless, it can be used only with certain types of applications
and customized applications. Applications that provide a read-only service, such
as DNS lookups, scale well with this model. In contrast, applications that modify
data, such as databases, must be modified to synchronize the modification activi-
ties across the cluster nodes.
Oracle Solaris Cluster implements this model via its Scalable Services function-
ality for web server software and some other applications. Oracle also provides
this capability for a database service with the Oracle RAC product.
Figure 1.7 shows two VEs in each of two computers that are part of a scalable
cluster. Each computer has two VEs providing the same service. This model is of-
ten used when the application does not scale well. After a failure, only one fourth
of the processing power of the cluster is lost.
Figure 1.7 Scalable Services Cluster
Summary In summary, consolidated systems provide clear benefits, but they also
require a return to some of the mainframe practices of the past, including more
assertive efforts to manage the resources and availability of the system. Using