Lvs Scheduling Overview; Scheduling Algorithms - Red Hat LINUX VIRTUAL SERVER 5.0 - ADMINISTRATION Manual

available database for transactions. Additionally, using an active-active configuration with Red
Hat Cluster Manager, administrators can configure one high-availability 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.

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. 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.

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. 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 assign
scheduling algorithms refer to
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.
Section 6.1, "The VIRTUAL SERVER
Real Servers
Subsection".
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