allowing the on-demand allocation of those resources to different partitions and the management of I/O devices. The physical resources are owned by the Virtual I/O server.

3.1.3 Why Logically Partition?

There is a demand to provide greater flexibility for high-end systems, particularly the ability to subdivide them into smaller partitions that are capable of running a version of an operating system or a specific set of application workloads.

The main reasons for partitioning a large system are as follows:

Server consolidation

A highly reliable server with sufficient processing capacity and capable of being partitioned can address the need for server consolidation by logically subdividing the server into a number of separate, smaller systems. This way, the application isolation needs can be met in a consolidated environment, with the additional benefits of reduced floor space, a single point of management, and easier redistribution of resources as workloads change. Increasing or decreasing the resources allocated to partitions can facilitate better utilization of a server that is exposed to large variations in workload.

Production and test environments

Generally, production and test environments should be isolated from each other. Without partitioning, the only practical way of performing application development and testing is to purchase additional hardware and software.

Partitioning is a way to set aside a portion of the system resources to use for testing new versions of applications and operating systems, while the production environment continues to run. This eliminates the need for additional servers dedicated to testing, and provides more confidence that the test versions will migrate smoothly into production because they are tested on the production hardware system.

Consolidation of multiple versions of the same OS or applications

The flexibility inherent in LPAR greatly aids the scheduling and implementation of normal upgrade and system maintenance activities. All the preparatory activities involved in upgrading an application or even an operating system could be completed in a separate partition. An LPAR can be created to test applications under new versions of the operating system prior to upgrading the production environments. Instead of having a separate server for this function, a minimum set of resources can be temporarily used to create a new LPAR where the tests are performed. When the partition is no longer needed, its resources can be incorporated back into the other LPARs.

Application isolation

Partitioning isolates an application from another in a different partition. For example, two applications on one symmetric multi-processing (SMP) system could interfere with each other or compete for the same resources. By separating the applications into their own partitions, they cannot interfere with each other. Also, if one application were to hang or crash the operating system, this would not have an effect on the other partitions. Also, applications are prevented from consuming excess resources, which could starve other applications of resources they require.

Increased hardware utilization

Partitioning is a way to achieve better hardware utilization when software does not scale well across large numbers of processors. Where possible, running multiple instances of an

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IBM DS8000 manual Why Logically Partition?, Server consolidation, Production and test environments, Application isolation