The impact of latency on brokering in the new zones of XenApp / XenDesktop 7.7

In this article the effect of latency on the brokering of connecting with the new zones introduced in XenApp / XenDesktop 7.7 is discussed. For the majority of end users, enumerating and launching resources is something they’ll do every day. With the addition of zones, Citrix is allowing users to be on higher latency links, so long as there’s a local broker.  With this additional latency, there will, inevitably, be an impact on end user experience. For the majority of work that users will do, they’ll see the expected slowness that’s linked to round-trips between the satellite brokers and the SQL database.

multi-geo blog diagrams

However, for launching Apps, there is a pain point in actually brokering sessions. This pain point is due to the need to pick the lowest-loaded VDA on which to launch an app. This occurs within a database transaction and needs a snapshot of all the current loads on the VDAs within the Delivery Group. To achieve this, a lock is taken out on all the workers in Delivery Group, which stops other users (IE causes serialization) from taking the same locks. It also waits on and blocks out worker state changes (eg. session events).

With low latency, the delay between taking the locks and releasing them is very small. However, as latency increases, so does the time the locks are held, and so the time to broker sessions increases.

To back this up, Citrix looked at a variety of latencies and launch rates. Round-trip times of 10ms cover most inter-country delays. 45ms covers North America, Europe and Intra-Japan; 90ms covers Trans-Atlantic; 160ms covers Trans Pacific, Latin America and Asia Pacific; finally, 250ms covers EMEA to Asia Pacific.

In summary: brokering does work over latency, but the latency needs to be considered for sizing a remote zone. If a zone is large, it may still be desirable to keep a database local to that zone. If the zone is small, using a remote zone may work well and might also reduce management cost without impacting on the end user experience.



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