I recommend you to check hotfixes for your version of Presentation server as some bugs related to election are fixed. I recommend you to check in the Citrix site.
Regarding the Zone data collector election preferences and other information, please find the information below:
On the Set server's election preferences page, click Edit and select the ranking for the server by choosing from the following election options:
Most Preferred. The server is always the first choice to become the data collector. It is recommended that only one server per zone be given this setting.
Preferred. When electing a new data collector, XenApp elects the next collector from the Preferred servers if the Most Preferred server is not available.
Default Preference. The default setting for all servers. The next collector is selected from the Default servers if neither a Most Preferred server nor a Preferred server is available.
Not Preferred. Apply this setting to servers that you do not want to become the data collector for the zone. This setting means that this server becomes the data collector only when no servers are available with any of the other three settings (Most Preferred, Preferred, Default Preference).
Data Collectors and Elections
A data collector is an in-memory database that maintains dynamic information about the servers in the zone, such as server loads, session status, published applications, users connected, and license usage.
Data collectors receive incremental data updates and queries from servers within their zone. Data collectors relay information to all other data collectors in the farm. The data collector tracks, for example, which applications are available and how many sessions are running on each server in the zone. The data collector communicates this information to the data store on behalf of the other servers in the farm. By default, in farms that communicate indirectly with the data store, the first server in the farm functions as the data collector.
Farms determine the data collector according to what level the election preference is set for a server. By default, all servers joining the farm are configured as backup data collectors. When the zone’s data collector fails, a data collector election occurs and a backup data collector replaces the failed data collector.
If the data collector fails, existing and incoming sessions connected to other servers in the farm are not affected. The data collector election process begins automatically and a new data collector is elected almost instantaneously. Data collector elections are not dependent on the data store.
New Data Collector Election
When communications fail between data collectors or between a server and its data collector, the process to choose or elect a new data collector begins. For example:
The data collector for Zone 1 has an unplanned failure. If the data collector shuts down correctly, it triggers the election process as it shuts down.
The servers in Zone 1 detect the data collector failed and start the election process. The server set to the next highest election preference is elected as the new data collector.
The member servers start sending their information to the new data collector for Zone 1.
The new Zone 1 data collector replicates this information to the other data collectors in the farm.
Sometimes, you might decide to have a dedicated data collector after your farm is in production. In general, if users experience slow connection times due to high CPU utilization on the data collector, consider dedicating a server to act solely as the zone data collector.
Planning for Data Collectors
When planning for data collectors, consider:
If you need a dedicated data collector
If you do not need a dedicated data collector, which infrastructure services can share the same server
If you need a zone in each geographic region, which means that you need data collectors for those regions as well
To maintain consistent information between zones, data collectors relay information to all other data collectors in a farm, creating network traffic.
In general, data collector memory consumption increases as farm size increases. However, it is not significant. For example, the Independent Management Architecture service running on the data collector typically uses 300 MB on a 1000 server farm.
Likewise, CPU usage is not significant. A data collector hosted on a dual-processor server can support over 1000 servers in its zone. In general, CPU usage increases as the number of servers in a zone increases, the number of zones increases, and the number of users launching applications increases.
On most networks, Citrix recommends reducing the number of data collectors and zones. For example, if you have a farm with 100 servers in one location, Citrix recommends having one zone with a dedicated data collector (although you can have backup data collectors).
Citrix recommends installing XenApp on the server you want to host the data collector functionality and, after installing other member servers, configuring a server as the backup data collector.
