Chapter 3. Application considerations 97
3.3 HACMP
The following sections provide an overview of the benefits of using High Performance Switch
in an HACMP environment.
3.3.1 HACMP overview
HACMP is a tool for building UNIX-based mission-critical computing platforms. HACMP
software ensures that critical resources, such as applications, are available for processing.
HACMP has two major components: high availability (HA) and cluster multi-processing
(CMP).
The primary reason for creating HACMP clusters is to provide a highly available environment
for mission-critical applications. For example, an HACMP cluster could run a database server
program that services client applications. The clients send queries to the server program,
which responds to their requests by accessing a database stored on a shared external disk.
In an HACMP cluster, the applications are put under HACMP control to ensure the availability
of these applications. HACMP takes measures to ensure that the applications remain
available to client processes even if a component in a cluster fails. To ensure availability, in
case of a component failure, HACMP moves the application (along with resources that ensure
access to the application) to another node in the cluster.
3.3.2 Role of HACMP
HACMP helps you with each of the following:
The HACMP planning process and documentation include tips and advice on the best
practices for installing and maintaining a highly available HACMP cluster.
Once the cluster is operational, HACMP provides the automated monitoring and recovery
for all of the resources on which the application depends.
HACMP provides a full set of tools for maintaining the cluster while keeping the application
available to clients.
HACMP lets you:
Set up an HACMP environment using online planning worksheets to simplify the initial
planning and setup.
Ensure high availability of applications by eliminating single points of failure in an HACMP
environment.
Leverage high availability features available in AIX.
Manage how a cluster handles component failures.
Secure cluster communications.
Set up fast disk takeover for volume groups managed by the Logical Volume Manager
(LVM).
Manage event processing for an HACMP environment.
Monitor HACMP components and diagnose problems that may occur.
3.3.3 Physical components of an HACMP cluster
HACMP provides a highly available environment by identifying a set of resources essential to
uninterrupted processing, and by defining a protocol that nodes use to collaborate to ensure
98 An Introduction to the New IBM Eserver pSeries High Performance Switch
that these resources are available. HACMP extends the clustering model by defining
relationships among cooperating processors, where one processor provides the service
offered by a peer should the peer be unable to do so.
An HACMP cluster is made up of the following physical components:
Nodes
Shared external disk devices
Networks
Network interfaces
Clients
The HACMP software allows you to combine physical components into a wide range of
cluster configurations, providing you with flexibility in building a cluster that meets your
processing requirements. Figure 3-14 shows one example of an HACMP cluster. Other
HACMP clusters could look very different, depending on the number of processors, the
choice of networking and disk technologies, and so on.
Figure 3-14 Example HACMP cluster scenario
Nodes
Nodes form the core of an HACMP cluster. A node is a processor that runs both AIX and the
HACMP software. HACMP supports pSeries uniprocessor and symmetric multiprocessor
(SMP) systems (including LPARs) and the Scalable POWERparallel processor (SP) systems
as cluster nodes. To the HACMP software, an SMP system looks just like a uniprocessor.
SMP systems provide a cost-effective way to increase cluster throughput. Each node in the
cluster can be a large SMP machine, extending an HACMP cluster far beyond the limits of a
single system and allowing thousands of clients to connect to a single database.
In an HACMP cluster, up to 32 nodes cooperate to provide a set of services or resources to
other entities. Clustering these servers to back up critical applications is a cost-effective high
availability option. A business can use more of its computing power while ensuring that its
Private LAN
Disk buses
Public LAN1
Public LAN2
Clients
Shared disk space (protected)
Nodes

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