SNP0 - Snapshot: Runs the job queues used, among other things, for snapshot refreshes.
D000 - Dispatcher: Used with the multi-threaded server configuration.
Shared Server Process - Shared Server Process: Used with the multi-threaded server configuration, and not considered part of the instance.
QMN0 - Queue Monitor: Used with Oracle Advanced Queueing.
LCK0 - Lock: Used with parallel server for inter-instance locking.
CKPT - Checkpoint: Initiates and controls the checkpoint process.
ARC0 - Archiver: Copies filled redo log files to the archive log destination.
Database Instance Configurations
Oracle Database runs in either a single-instance configuration or an Oracle Real Application Clusters (Oracle RAC) configuration. These configurations are mutually exclusive. In a single-instance configuration, a one-to-one relationship exists between the database and a database instance as shown in figure 4-3.1.
In Oracle RAC, a one-to-many relationship exists between the database and database instances. The following figure 4-3.2 shows database instance configuration for RAC.
Whether in a single-instance or Oracle RAC configuration, a database instance is associated with only one database at a time. You can start a database instance and mount (associate the instance with) one database, but not mount two databases simultaneously with the same instance