Red Hat Enterprise Linux 5 Cluster Suite Overview page 31

Service requests arriving at an LVS router are addressed to a virtual IP address or VIP. This is a
publicly-routable address that the administrator of the site associates with a fully-qualified domain
name, such as www.example.com, and which is assigned to one or more virtual servers
VIP address migrates from one LVS router to the other during a failover, thus maintaining a presence
at that IP address, also known as floating IP addresses.
VIP addresses may be aliased to the same device that connects the LVS router to the public network.
For instance, if eth0 is connected to the Internet, then multiple virtual servers can be aliased to
eth0:1. Alternatively, each virtual server can be associated with a separate device per service. For
example, HTTP traffic can be handled on eth0:1, and FTP traffic can be handled on eth0:2.
Only one LVS router is active at a time. The role of the active LVS router is to redirect service requests
from virtual IP addresses to the real servers. The redirection is based on one of eight load-balancing
algorithms:
• Round-Robin Scheduling — Distributes each request sequentially around a pool of real servers.
Using this algorithm, all the real servers are treated as equals without regard to capacity or load.
• Weighted Round-Robin Scheduling — Distributes each request sequentially around a 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 up or down by dynamic load information. This is a
preferred choice if there are significant differences in the capacity of real servers in a server pool.
However, if the request load varies dramatically, a more heavily weighted server may answer more
than its share of requests.
• Least-Connection — Distributes more requests to real servers with fewer active connections. This
is a type of dynamic scheduling algorithm, making it a better choice if there is a high degree of
variation in the request load. It is best suited for a real server pool where each server node has
roughly the same capacity. If the real servers have varying capabilities, weighted least-connection
scheduling is a better choice.
• 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 up or down by dynamic load information. The addition of weighting makes this
algorithm ideal when the real server pool contains hardware of varying capacity.
• Locality-Based Least-Connection Scheduling — Distributes more requests to servers with fewer
active connections relative to their destination IPs. This algorithm is 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 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.
A virtual server is a service configured to listen on a specific virtual IP.
Two-Tier LVS Topology
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