In a clustered environment,by default monitoring tables report on a per-instance (that is, a single server in the cluster) basis instead of returning cluster-wide results. This allows you to monitor the activities of processes and queries across the cluster to get a better understanding of the statistics for objects that may be opened on more than one instance, and resource usage on each instance in the cluster. For example, if you query the monitoring tables about a table, the table about which you are querying may be opened or accessed by more than one instance in the cluster, so the descriptors for this table—and the associated statistics—may be in memory on the instance. Statistics are not aggregated for the cluster. The statistical results for all instances are returned as a unioned result set with rows collected from each instance. Each instance is identified in the result set with a row in the InstanceID column.
