10 ServerIron ADX Global Server Load Balancing Guide
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Global Server Load Balancing overview
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Site ServerIron ADX’s connection loadA GSLB site’s connection load is the average number of new connections per second on the site,
over a given number of intervals. When you enable this GSLB metric, all potential candidates are
compared against a predefined load limit. All sites that have fewer average connections than the
threshold are selected and passed to the next comparison metric. The connection limit metric is
disabled by default but is enabled (added to the GSLB policy) when you configure the metric.
Site ServerIron ADX’s available session capacity toleranceIf multiple sites are equal after the above comparisons, the GSLB ServerIron ADX prefers the site
ServerIron ADX (remote ServerIron ADX) whose session table has the most unused entries.
When comparing sites based on the session table utilization, the GSLB ServerIron ADX considers
the sites to be equal if the difference in session table utilization does not exceed the tolerance
percentage. The tolerance percentage ensures that minor differences in utilization do not cause
frequent, and unnecessary, changes in site preference.
For example, suppose one ServerIron ADX has 1 million sessions available, and another has
800,000 sessions available. Also assume that the tolerance is 10% (the default). In this case the
first ServerIron ADX (with 1 million sessions available) is preferred over the second ServerIron ADX
because the difference (200,000) is greater than 10% of 1 million. If a third ServerIron ADX has
950,000 sessions available, that ServerIron ADX is equally preferable with the first ServerIron ADX
(with 1 million sessions available), because the difference in percentage between the available
sessions on the two ServerIron ADXs is only 5%, which is less than the tolerance threshold.
Site ServerIron ADX’s FlashBack speedIf multiple sites compare equally based on all the metrics above, the ServerIron ADX chooses a site
as the best one based on how quickly the GSLB ServerIron ADX received responses to health
checks to the site ServerIron ADX.
The GSLB ServerIron ADX uses a tolerance value when comparing the FlashBack speeds of
different sites. The tolerance value specifies the percentage by which the FlashBack speeds of the
two sites must differ in order for the ServerIron ADX to choose one over the other. The default
FlashBack tolerance is 10%. Thus, if the FlashBack speeds of two sites are within 10% of one
another, the ServerIron ADX considers the sites to be equal. However, if the speeds differ by more
than 10%, the ServerIron ADX prefers the site with the lower FlashBack speed.
FlashBack speeds are measured at Layer 4 for all TCP/UDP ports. For the application ports known
to the ServerIron ADX, the FlashBack speed of the application is also measured.
When the ServerIron ADX compares the FlashBack speeds, it compares the Layer 7
(application-level) FlashBack speeds first, if applicable. If the application has a Layer 7 health
check and if the FlashBack speeds are not equal, the ServerIron ADX is through comparing the
FlashBack speeds. If a host is associated with multiple applications, the GSLB ServerIron ADX uses
the slowest response time among the applications for the comparison. However, if only the Layer 4
health check applies to the application, or if further tie-breaking is needed, the ServerIron ADX then
compares the Layer 4 FlashBack speeds.