The Managed Global Area (MGA) is a unique memory framework that has the capability to share and coordinate memory across a set of trusted Oracle processes.
It has the capability to be a configurable, elastic, modular, on-demand, recoverable shared memory area. MGA provides a way to utilize different types of shared memory areas in the database. It is exposed as namespaces for internal database layers, each belonging to only
one of them. A namespace primarily consists of set of shared memory segments and metadata that goes along with it.
The layers in the database may define the namespace’s characteristics, scope, lifetime, and internal segment size. All MGA is accounted under the PGA aggregate limit.
On Linux, MGA requires tempfs mounted on /dev/shm and is configured to at least the size of PGA aggregate limit. When multiple instances are running on the same machine, the size of /dev/shm needs to be at least sum(PGA_AGGREGATE_LIMIT) across all the instances.
