Enterprise database servers can use all available memory due to too many open sessions or runaway workloads. Running out of memory can result in failed transactions or, in extreme cases, a restart of the server and the loss of a valuable resource for your applications. Memory Guard detects memory pressure on a server in real time and redirects new sessions to other servers to prevent using all available memory on the stressed server.
When Oracle Database QoS Management is enabled and managing an Oracle Clusterware server pool, Cluster Health Monitor sends a metrics stream that provides real-time information about memory resources for the cluster servers to Memory Guard. This information includes the following:
Amount of available memory
Amount of memory currently in use
If Memory Guard determines that a node is experiencing memory pressure, then the database services managed by Oracle Clusterware are stopped on that node, preventing new connections from being created. After the memory stress is relieved, the services on that node are restarted automatically, and the listener starts sending new connections to that server. The memory pressure can be relieved in several ways (for example, by closing existing sessions or by user intervention).
Rerouting new sessions to different servers protects the existing workloads on the memory-stressed server and enables the server to remain available. Memory Guard is a feature of Oracle RAC that manages the memory pressure for servers, adding a new resource protection capability in managing service levels for applications hosted on Oracle RAC databases.