Brocade Virtual ADX Global Server Load Balancing Guide (Supporting ADX v03.1.00) - Manual (Page 20)

10 Brocade Virtual ADX Global Server Load Balancing Guide
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Global Server Load Balancing overview
1
Site ADX device’s available session capacity tolerance
If multiple sites are equal after the above comparisons, the GSLB Brocade Virtual ADX prefers the
site ADX device (remote ADX device) whose session table has the most unused entries.
When comparing sites based on the session table utilization, the GSLB Brocade Virtual 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 ADX device 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 ADX
device (with 1 million sessions available) is preferred over the second ADX device because the
difference (200,000) is greater than 10% of 1 million. If a third ADX device has 950,000 sessions
available, that ADX device is equally preferable with the first ADX device (with 1 million sessions
available), because the difference in percentage between the available sessions on the two ADX
devices is only 5%, which is less than the tolerance threshold.
Site ADX device’s FlashBack speed
If multiple sites compare equally based on all the metrics above, the Brocade Virtual ADX chooses
a site as the best one based on how quickly the GSLB Brocade Virtual ADX received responses to
health checks to the site ADX device.
The GSLB Brocade Virtual 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 Brocade Virtual 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 Brocade Virtual ADX considers the sites to be equal. However, if the speeds differ by
more than 10%, the Brocade Virtual 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 Brocade Virtual ADX, the FlashBack speed of the application is also measured.
When the Brocade Virtual 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 Brocade Virtual ADX is through comparing the
FlashBack speeds. If a host is associated with multiple applications, the GSLB Brocade Virtual 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 Brocade
Virtual ADX then compares the Layer 4 FlashBack speeds.