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Understanding The New Logon Throttling Load Evaluator Options In Citrix Presentation Server 4.5

Sunday, 06 May 2007 by Michel Roth

Brian Madden has written an article which explains how you configure the new logon throttling load evaluator options in Citrix Presentation Server 4.5. The article also details how these options work and also how you can deal with the "Black Hole Effect" in earlier versions of Citrix. You do know that you can prevent the black hole effect on just RDP servers as well by using DADE power tools, right?

"Load balancing in a Citrix Presentation Server environment has not fundamentally changed since the days of MetaFrame XP. Today's PS 4.5 environments still have zone data collectors, load evaluators, and load indexes.
...This load balancing architecture generally works fine but has one fatal flaw: new incoming requests are always routed to the least-busy server. While this doesnt seem like a problem at first, consider an environment that has maybe 20 servers each with 80 users. If a new server is brought online (or if a server unexpectedly reboots), that new server will have 0 sessions while the other servers all have 80 sessions. So guess which server the next 80 users will be routed to?

...What this means is that a new server coming online during a busy period can essentially prevent all new logons across your entire farm! This is known colloquially as the black hole effect.

Citrix of course is aware of this problem. When the current generation of load balancing technology was released in MetaFrame XP, Citrix attempted to mitigate this with something called the load bias. The load bias was an temporary and artificial increase of a servers load index that happened whenever a server received an new incoming connection."

Read the article here.