Server Weight And Scheduling - Red Hat ENTERPRISE LINUX 5 - VIRTUAL SERVER ADMINISTRATION Manual

Chapter 1. Linux Virtual Server Overview
Weighted Least-Connections (default)
Distributes more requests to servers with fewer active connections relative to their capacities.
Capacity is indicated by a user-assigned weight, which is then adjusted upward or downward
by dynamic load information. The addition of weighting makes this algorithm ideal when the real
server pool contains hardware of varying capacity. Refer to
Scheduling"
for more on weighting real servers.
Locality-Based Least-Connection Scheduling
Distributes more requests to servers with fewer active connections relative to their destination IPs.
This algorithm is designed for use in a proxy-cache server cluster. It routes the packets for an IP
address to the server for that address unless that server is above its capacity and has a server in
its half load, in which case it assigns the IP address to the least loaded real server.
Locality-Based Least-Connection Scheduling with Replication Scheduling
Distributes more requests to servers with fewer active connections relative to their destination
IPs. This algorithm is also designed for use in a proxy-cache server cluster. It differs from Locality-
Based Least-Connection Scheduling by mapping the target IP address to a subset of real
server nodes. Requests are then routed to the server in this subset with the lowest number of
connections. If all the nodes for the destination IP are above capacity, it replicates a new server
for that destination IP address by adding the real server with the least connections from the overall
pool of real servers to the subset of real servers for that destination IP. The most loaded node is
then dropped from the real server subset to prevent over-replication.
Destination Hash Scheduling
Distributes requests to the pool of real servers by looking up the destination IP in a static hash
table. This algorithm is designed for use in a proxy-cache server cluster.
Source Hash Scheduling
Distributes requests to the pool of real servers by looking up the source IP in a static hash table.
This algorithm is designed for LVS routers with multiple firewalls.

1.3.2. Server Weight and Scheduling

The administrator of LVS can assign a weight to each node in the real server pool. This weight is an
integer value which is factored into any weight-aware scheduling algorithms (such as weighted least-
connections) and helps the LVS router more evenly load hardware with different capabilities.
Weights work as a ratio relative to one another. For instance, if one real server has a weight of 1 and
the other server has a weight of 5, then the server with a weight of 5 gets 5 connections for every 1
connection the other server gets. The default value for a real server weight is 1.
Although adding weight to varying hardware configurations in a real server pool can help load-balance
the cluster more efficiently, it can cause temporary imbalances when a real server is introduced to the
real server pool and the virtual server is scheduled using weighted least-connections. For example,
suppose there are three servers in the real server pool. Servers A and B are weighted at 1 and
the third, server C, is weighted at 2. If server C goes down for any reason, servers A and B evenly
distributes the abandoned load. However, once server C comes back online, the LVS router sees it
has zero connections and floods the server with all incoming requests until it is on par with servers A
and B.
To prevent this phenomenon, administrators can make the virtual server a quiesce server — anytime
a new real server node comes online, the least-connections table is reset to zero and the LVS router
routes requests as if all the real servers were newly added to the cluster.
6
Section 1.3.2, "Server Weight and