Avaya P333R-LB manual Hash Metric Loss of Persistency

Chapter 14 Load Balancing in the P333R-LB

Figure 14.16 Hash Metric - Loss of Persistency

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Hash Function	3	The same
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		Hash Function
pointing to Server 4	1
		pointing to Server 1
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List of available Servers

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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

14.17illustrates how persistency is maintained even though a cache server becomes non-operational.

Figure 14.17 MinMiss Hash Metric - Persistency Sustained

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Hash Function	3	The same
	4	Hash Function still
pointing to Server 4	1	pointing to Server 4
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List of available Servers

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Persistency

Kept

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

Avaya P333R-LB User’s Guide 53