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Chapter 14 Load Balancing in the P333R-LB
Avaya P333R-LB User’s Guide 53
Figure 14.16 Hash Metric - Loss of Persistency
When Cache Server 2 in Figure 14.16 is removed from the group, the list of available
cache servers is readjusted, causing a lack of persistency. However, if Cache Server
2 becomes operational again, the list of available cache servers is restored to its
original order, and persistency is recovered.
MinMiss Hash
The default metric parameter for MinMiss Hash in AR is the Destination IP address.
MinMiss distributes sessions to cache servers in the same way as the Hash metric.
However, MinMiss Hash sustains persistency even when a cache server is removed
from the cache server group. When a cache server fails or is removed, the positions
of the other cache servers in the list do not change. Instead, the remaining cache
servers are redistributed to the list entries freed by the failing cache server. Figure
14.17 illustrates how persistency is maintained even though a cache server becomes
non-operational.
Figure 14.17 MinMiss Hash Metric - Persistency Sustained
When Cache Server 2 in Figure 14.17 is removed from the group, the list of available
cache servers is not readjusted. Only the list entries that are now empty are replaced
with other available cache servers. Therefore, persistency is sustained for all
available cache servers. If Cache Server 2 becomes operational again, the list of
available cache servers is recalculated so that the smallest number of cache servers is
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Loss of
Persistency
Hash Function
pointing to Server 4
The same
Hash Function
pointing to Server 1
List of available Servers
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Server 2
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Persistency
Kept
Hash Function
pointing to Server 4
The same
Hash Function still
pointing to Server 4
List of available Servers