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“main” (Installation and Administration) — 2004/6/25 — 13:29 — page 399 — #425

High Availability under Linux

19.5 Clustering

19.5.1 Cluster Alias

The cluster alias is a technology that allows several nodes to be conﬁgured

with a shared IP address, while also permitting TCP/IP connections to be

established at this address. Inbound TCP/IP connections are automatically

distributed.

Unlike the Linux virtual server, a dedicated load balancer is not required.

However, because of the type of implementation, the cluster alias is less

efﬁcient when there is a large number of nodes. In the case of the cluster

alias, all IP packages are distributed to all nodes, which then ﬁlter out the

packages intended for them. In the case of LVS, this decision is only taken

once by the load balancer. For further information about how to conﬁgure

this feature, see the iptables manual page.

19.5.2 Linux Virtual Server

Linux virtual server is based on a real cluster of several servers, which are

connected together by means of a load balancer for distributing the load

among the various members of the cluster. From the outside, a cluster of

this kind simply looks like a single virtual server. The load balancer should

also be conﬁgured for redundancy and should be secured using heart-

beat. The aim of an LVS conﬁguration is to make the best possible use

of the existing resources and to offer good scalability. The heartbeat-

ldirectord daemon is used in these scenarios to monitor the “health” of

the various real servers.

19.5.3 High Availability Clusters

High availability clusters are designed so all available services can be pro-

vided at all times, despite hardware or software failures. If a node in a clus-

ter fails, another takes over immediately. This node (secondary) is a mirror

image of the failed node (primary) and actually assumes the identity of the

failed node during failover, so the cluster environment remains externally

consistent.

399SUSE LINUX Enterprise Server