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Chapter 7. Linux Virtual Server Overview
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.
7.3.2. 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 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. How-
ever, 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.
7.4. Routing Methods
Red Hat Enterprise Linux uses Network Address Translation or NAT routing for LVS clus-
tering, which allows the administrator tremendous flexibility when utilizing available hard-
ware and integrating the cluster into an existing network.
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