Maintaining High Availability with True Cache
Use True Cache in high availability mode to keep critical and frequently accessed data available for application reads during primary database outages, whether planned or unplanned. High availability mode also improves performance during normal operation.
Read Availability During Primary Database Outages
During normal operation, True Cache fetches missing data and metadata blocks from the primary database and maintains consistent cached data as the primary database changes.
When the primary database becomes unavailable, True Cache continues serving reads using the cached data. If a query requires data blocks that aren’t cached, it returns ORA-61857, indicating failed communication with the primary database.
High availability mode automatically caches the metadata needed to serve application reads during a primary database outage. To prepare the application data needed for critical queries, you can warm up the cache by running those queries on True Cache before an outage occurs.
Performance Benefits
Applications with high data modification rates can benefit significantly from enabling True Cache high availability mode, even while the primary database is available.
In high availability mode, True Cache proactively retrieves and caches undo, undo segment headers, dictionary, metadata, and certain hot object blocks from the primary database. Undo blocks are maintained in the cache as long as they are useful (that is, as long as the undo block SCN is above the True Cache minimum active SCN).
Without this proactive retrieval, missing undo blocks can cause repeated cache misses that require True Cache to fetch blocks from the primary database during query processing.
By reducing these fetches, high availability mode can significantly improve application read performance for data that has been recently modified on the primary database.
Memory Requirement
To enable high availability mode, on the True Cache node set the SGA_TARGET to at least 256 GB.
If SGA_TARGET is not specified on the True Cache node, the corresponding threshold is determined by DB_CACHE_SIZE = 175GB.