Red Hat ENTERPRISE LINUX AS 2.1 -, Installation Manual

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Section 6.4:Routing Methods 107
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.
6.3.2 Server Weight and Scheduling
The administrator of an LVS cluster 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 load hardware with different capabilities more
evenly.
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 will get 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. To illustrate, let
us say 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 some reason, servers A and B will take up the
slack. But once server C comes back online, the LVS router will see it has zero connections and flood
the server with all incoming requests until it is on par with servers A and B.
To prevent this phenomenon, the administrator can make the virtual server a quiesce server — anytime
a new real server node comes online, the least-connections table is reset to zero so the LVS router routes
requests as if all the real servers were freshly added to the cluster.
6.4 Routing Methods
Red Hat Enterprise Linux AS 2.1 uses Network Address Translation or NAT routing for LVS clus-
tering — affording the administrator tremendous flexibility when utilizing available hardware and
integrating the cluster into an existing network.