Database Cache Size Effective (MB) is the amount of memory (in megabytes) reserved for fully dehydrated page data by the ...

Database Cache Size Effective (MB) is the amount of memory (in megabytes) reserved for fully  dehydrated page data by the database cache manager.  This is not the actual system memory used, but the hypothetical memory usage in effect if all  dehydrated/ compressed database cache buffers were rehydrated/uncompressed.