Red Hat ENTERPRISE LINUX AS 2.1 Installation Manual page 99

Section 5.2:Basic Configurations 99
Having access to the same data source helps Red Hat Cluster Manager more effectively handle failover
situations because after a failure occurs the functional node's newly activated services have access to
the exact same data used by the failed node. However, to protect the integrity of data on shared devices,
services within a high-availability cluster are only allowed to run on one node at any given time.
Red Hat Cluster Manager's use of shared storage also gives administrators flexibility in how they
use each node in the cluster. For example, one can either run different services on each server — a
configuration known as active-active — or run all services on one node while the other sits idle — a
configuration known as hot-standby.
The shared storage device in a Red Hat Cluster Manager cluster also enables each node to verify
the health of the other by regularly updated status information on mutually accessible quorum disk
partitions. If the quorum partition is not updated properly by a member of the cluster, the other node
can verify the integrity of that member by pinging it through a heartbeat channel. Heartbeat channels
can be configured on one or more Ethernet interfaces or a serial connection or on both interfaces
concurrently.
For more information about configuring Red Hat Cluster Manager clusters, please see the accompa-
nying manual titled Red Hat Cluster Manager Installation and Administration Guide.
5.2.2 Load-Balancing Clusters Using Linux Virtual Servers
To the outside world, an LVS cluster appears as one server, but in reality, a user from the World Wide
Web who is accessing a group of servers behind a pair of redundant LVS routers.
An LVS cluster consists of at least two layers. The first layer is composed of a pair of similarly con-
figured Linux machines or nodes. One of these nodes acts as an LVS router, directing requests from
the Internet to the second layer — a pool of servers called real servers. The real servers provide the
critical services to the end-user while the LVS router balances the load to these servers.
For a detailed overview of LVS clustering, see Chapter 6, Linux Virtual Server Overview.