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1.3. LVS Scheduling Overview

One of the advantages of using LVS is its ability to perform flexible, IP-level load balancing on the real

server pool. This flexibility is due to the variety of scheduling algorithms an administrator can choose

from when configuring LVS. LVS load balancing is superior to less flexible methods, such as Round-

Robin DNS where the hierarchical nature 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, optionally, an

administrator-assigned weight factor when routing service requests. Using assigned weights gives

arbitrary priorities to individual machines. Using this form of scheduling, 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 LVS 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 address.

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

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. T he IPVS table is constantly updated by a utility called ipvsadm — adding

and removing cluster members depending on their availability.

1.3.1. Scheduling Algorithms

The structure that the IPVS table takes depends on the scheduling algorithm that the administrator

chooses for any given virtual server. T o 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 assign scheduling algorithms refer to

Section 4.6.1, “The VIRTUAL SERVER Subsection”.

Round-Robin Scheduling

Distributes each request sequentially around the pool of real servers. Using this algorithm, 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 that it is network-connection

based and not host-based. LVS round-robin scheduling also does not suffer the imbalances

caused by cached DNS queries.

Weighted Round-Robin Scheduling

Distributes each request sequentially around the pool of real servers but gives more jobs to

servers with greater capacity. Capacity is indicated by a user-assigned weight factor, which is

then adjusted upward or downward by dynamic load information. Refer to Section 1.3.2, “Server

Weight and Scheduling” for more on weighting real servers.

Weighted round-robin scheduling is a preferred choice if there are significant differences in the

capacity of real servers in the pool. However, if the request load varies dramatically, the more

heavily weighted server may answer more than its share of requests.

Least-Connection

Distributes more requests to real servers with fewer active connections. Because it keeps track

of live connections to the real servers through the IPVS table, least-connection is a type of

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