Avaya P333R-LB - page 181

Chapter 14 Load Balancing in the P333R-LB
Avaya P333R-LB User’s Guide 39
Figure 14.12 Hash Metric - Loss of Persistency
When Server 2 is removed from the group, the list of available servers is readjusted,
causing a lack of persistency. However, if Server 2 becomes operational again, the
list of available servers is restored to its original order, and persistency is recovered.
MinMiss Hash
MinMiss Hash distributes sessions to Real Servers in the same way as the Hash
metric. However, MinMiss Hash sustains persistency even when a server is
removed from the server group. When a server fails or is removed, the positions of
the other Real Servers in the list do not change. Instead, the remaining servers are
redistributed to the list entries freed by the failing server. The following figure
illustrates how persistency is maintained even though a server becomes non-
operational.
Figure 14.13 MinMiss Hash Metric - Persistency Sustained
When Server 2 is removed from the group, the list of available servers is not
readjusted. Only the list entries that are now empty are replaced with other
available servers. Therefore, persistency is sustained for all available servers. If
Server 2 becomes operational again, the list of available servers is recalculated so
that the smallest number of servers is affected. However, the list of servers is not
restored to its original configuration. As a result, persistency is only partially
recovered.
1
2
3
4
1
2
3
4
1
2
3
Server
Server 2
1
3
4
1
3
4
1
3
4
1
3
Loss of
Persistency
Hash Function
pointing to Server 4
The same
Hash Function
pointing to Server 1
List of available Servers
1
2
3
4
1
2
3
4
1
2
3
Server
Server 2
1
1
3
4
1
3
3
4
1
4
3
Persistency
Kept
Hash Function
pointing to Server 4
The same
Hash Function still
pointing to Server 4
List of available Servers