This article contains information about Load Balancing Slow Start mode on a NetScaler appliance.

Slow Start Mode

The virtual server on a NetScaler appliance gets into a Slow Start mode or a Startup Round Robin mode whenever a new service is enabled or a new service is added to the farm. The load balancing algorithm falls back to Round Robin method regardless of the configured algorithm on the virtual server. 

Least Connections is the default load balancing method. When configured, the appliance selects the service that has the least number of connections. For example, if the Least Connections method is in use and a new server is added to the server farm, the load balancing algorithm can cause the new server to be overloaded with requests, because it has fewer existing connections than other servers in the farm. To avoid overloading of servers, the appliance performs Slow Start. During this phase, the appliance distributes the requests by the Round Robin method regardless of the actual method configured.

The Slow Start mode functionality is available only for virtual servers that use one of the following load balancing methods:

• Least Connections
• Least Response Time
• Least Bandwidth
• Least Packets

Slow Start mode is triggered when one of the following conditions are true:

• Load balancing method changes to one of the methods mentioned in the preceding list.
• A new service is bound to the virtual server.
• When a service changes its state from DOWN to UP.
• When a service bound to the virtual server is enabled.

Slow Start Calculation

For a virtual server that is already configured and is serving the production traffic, when the services are enabled or the services are UP, the time to exit Slow Start is calculated using the following calculation:
Request rate = current instance value - previous instance value (before 7 seconds)

If the appliance has seven packet engines with 10 services bound to the virtual server, and the request rate is 100 per second, then the virtual server exits the Slow Start mode when it reaches 100 hits x number of packet engines (7) x bound services (10) = 7000 hits

Notes:

• As soon as one of the packet engines gets 1000 hits for that virtual server, the virtual server will be out of the Round Robin method and broadcasts the message to all other packet engines. Even if all other packet engines have not yet received the 1000 hits, it will still be out of the Round Robin method.

• The number of services bound also include the services which are in DOWN or OFS state.

Startup RR Factor

The appliance can alternatively be configured to require that a specific given number of requests should pass through the virtual server before exiting the Slow Start mode. Run the following command to set this configuration by using the Startup RR Factor:
> set lbparameter startupRRFactor 5

For more information refer to Citrix Documentation - Load Balancing Algorithms

If the appliance has seven packet engines with 10 services bound to the virtual server and the startup_rr_factor is 5, the virtual server exits the Slow Start mode when it reaches the following:
5 hits x bound services (10) x number of packet engines (7) = 350 hits (max)

Note: As soon as one of the packet engine gets 50 hits for that virtual server, it comes out of the Round Robin mode and broadcasts the message to all other packet engines. Even if all other packet engines have not yet received the 50 hits, it will still come out of the Round Robin method.

By default the newly configured virtual server remains in a Slow Start mode for Startup RR Factor of 100.

Additional Resources

Citrix Documentation - Gradually Stepping Up the Load on a New Service with Virtual Server–Level Slow Start.

Citrix Blog - Fast Ramp, Slow Start and Endpoint modes