HP Traffic Director sa7220 - Manual (Page 35)

C H A P T E R 2 Load Balancing Across Multiple Servers
23
Load
Balancing
Across
Multiple
Servers Balancing Algorithms
The SA8220 provides a choice of load balancing algorithms. Services
can be separately configured to load balance using a round-robin or a
response time algorithm. In most networks, the best performance
results from use of the response time algorithm. Under this algorithm,
the SA8220 measures the response time of each request to each server
in the server farm. It then balances requests to the service among the
servers, sending more requests to the fastest servers and fewer to the
slower ones, thus optimizing the average response time.
In cases where Out-of-Path Return (please see “Out-of-Path Return
(OPR) in Chapter 2) is used in unpredictable WAN environments,
response time metrics may be obscured by WAN latency variance. In
these situations, round-robin load balancing can provide equal
distribution of client requests to each fulfillment server.
The balancing algorithm is specified with the command:
config policygroup <name> service <name>
balancing [robin | load]
Response-Time Metrics
For both balancing algorithms, servers can be assigned target
response times. These values indicate the desired average response
time for requests for specified services to be fulfilled, and instructs
the SA8220 to use alternate resources for fulfillment if the average
response time exceeds target response time. Target response time is
controlled with the following command:
config policygroup <name> service <name>
response <mil-seconds>
If the servers do not meet the specified response time threshold,
backup servers, if available and enabled, are activated. In addition,
the servers providing lower priority services are throttled if the
response time is still not being met (if
throttle is enabled in the
policygroup). Both mechanisms are available for both of the load-
balancing algorithms.