Installation guide

Chapter 7. Linux Virtual Server Overview 89

cluster to serve both of these roles simultaneously.

The third tier in the above example does not have to use Red Hat Cluster Manager, but

failing to use a highly available solution would introduce a critical single point of failure.

7.3. LVS Scheduling Overview

One of the advantages of using an LVS cluster is its ability to perform ﬂexible, IP-level load

balancing on the real server pool. This ﬂexibility is due to the variety of scheduling algo-

rithms an administrator can choose from when conﬁguring a cluster. LVS load balancing

is superior to less ﬂexible methods, such as Round-Robin DNS where the hierarchical na-

ture of DNS and the caching by client machines can lead to load imbalances. Additionally,

the low-level ﬁltering employed by the LVS router has advantages over application-level

request forwarding because balancing loads at the network packet level causes minimal

computational overhead and allows for greater scalability.

Using scheduling, the active router can take into account the real servers’ activity and, op-

tionally, an administrator-assigned weight factor when routing service requests. Using as-

signed weights gives arbitrary priorities to individual machines. Using this form of schedul-

ing, it is possible to create a group of real servers using a variety of hardware and software

combinations and the active router can evenly load each real server.

The scheduling mechanism for an LVS cluster is provided by a collection of kernel patches

called IP Virtual Server or IPVS modules. These modules enable layer 4 (L4) transport

layer switching, which is designed to work well with multiple servers on a single IP ad-

dress.

To track and route packets to the real servers efﬁciently, IPVS builds an IPVS table in the

kernel. This table is used by the active LVS router to redirect requests from a virtual server

address to and returning from real servers in the pool. The IPVS table is constantly updated

by a utility called ipvsadm — adding and removing cluster members depending on their

availability.

7.3.1. Scheduling Algorithms

The structure that the IPVS table takes depends on the scheduling algorithm that the admin-

istrator chooses for any given virtual server. To allow for maximum ﬂexibility in the types

of services you can cluster and how these services are scheduled, Red Hat Enterprise Linux

provides the following scheduling algorithms listed below. For instructions on how to as-

sign scheduling algorithms refer to Section 10.6.1 The VIRTUAL SERVER Subsection.

Round-Robin Scheduling

Distributes each request sequentially around the pool of real servers. Using this algo-

rithm, all the real servers are treated as equals without regard to capacity or load. This

scheduling model resembles round-robin DNS but is more granular due to the fact