摘自：Linux企业集群(The Linux Enterprise Cluster)英文原文: The Linux Enterprise ClusterOverviewThis chapter will introduce the cluster load-balancing software called IP Virtual Server (IPVS). The IPVS software is a collection of kernel patches that were merged into the stock version of the Linux kernel starting with version 2.4.23. When combined with the kernels routing and packet-filtering capabilities (discussed in Chapter 2) the IPVS-enabled kernel lets you turn any computer running Linux into a cluster load balancer. Together, the IPVS-enabled cluster load balancer and the cluster nodes are called a Linux Virtual Server (LVS).The LVS cluster load balancer accepts all incoming client computer requests for services and decides which cluster node should reply to each request. The load balancer is sometimes called an LVS Director or simply a Director. In this book the terms LVS Director, Director, and load balancer all refer to the same thing.The nodes inside an LVS cluster are called real servers, and the computers that connect to the cluster to request its services are called client computers. The client computers, the Director, and the real servers communicate with each other using IP addresses the same way computers have always exchanged packets over a network; however, to make it easier to discuss this network communication, the LVS community has developed a naming convention to describe each type of IP address based on its role in the network conversation. So before we consider the different types of LVS clusters and the choices you have for distributing your workload across the cluster nodes (called scheduling methods), lets look at this naming convention and see how it helps describe the LVS cluster.LVS IP Address Name ConventionsIn an LVS cluster, we cannot refer to network addresses as simply IP addresses. Instead, we must distinguish between different types of IP addresses based on the roles of the nodes inside the cluster. Here are four basic types of IP addresses used in a cluster:Virtual IP (VIP) addressThe IP address the Director uses to offer services to client computersReal IP (RIP) addressThe IP address used on the cluster nodesDirectors IP (DIP) addressThe IP address the Director uses to connect to the D/RIP networkClient computers IP (CIP) addressThe IP address assigned to a client computer that it uses as a source IP address for requests sent to the clusterThe Virtual IP (VIP)The IP address that client computers use to connect to the services offered by the cluster are called virtual IP addresses (VIPs). VIPs are IP aliases or secondary IP addresses on the NIC that connects the Director to the normal, public network.1 The LVS VIP is important because it is the address that client computers will use when they connect to the cluster. Client computers send packets from their IP address to the VIP address to access cluster services. You tell the client computers the VIP address using a naming service (such as DNS, DDNS, WINS, LDAP, or NIS), and this is the only name or address that client computers ever need to know in order to use the services inside the cluster. (The remaining IP addresses inside the cluster are not known to the client computer.)A single Director can have multiple VIPs offering different services to client computers, and the VIPs can be public IP addresses that can be routed on the Internet, though this is not required. What is required, however, is that the client computers be able to access the VIP or VIPs of the cluster. (As well see later, an LVS-NAT cluster can use a private intranet IP address for the nodes inside the cluster, even though the VIP on the Director is a public Internet IP address.)The Real IP (RIP)In LVS terms, a node offering services to the outside world is called a real server. (We will use the terms cluster node and real server interchangeably throughout this book.) The IP address used on the real server is therefore called a real IP address (RIP).The RIP address is the IP address that is permanently assigned to the NIC that connects the real server to the same network as the Director. Well call this network cluster network or the Director/real-server network (D/RIP network). The Director uses the RIP address for normal network communication with the real servers on the D/RIP network, but only the Director needs to know how to talk to this IP address.The Directors IP (DIP)The Directors IP (DIP) address is used on the NIC that connects the Director to the D/RIP network. As requests for cluster services are received on the Directors VIP, they are forwarded out the DIP to reach a cluster node. As is discussed in Chapter 15, the DIP and the VIP can be on the same NIC.The Client Computers IP (CIP)The client computers IP (CIP) address may be a local, private IP address on the same network as the VIP, or it may be a public IP address on the Internet.Types of LVS ClustersNow that weve looked at