An Oracle instance is a memory region in the computer and a set of executing processes. The SGA memory is allocated within the OS environment at database start time, and may become paged-out if necessary.
The main database processes are started within the OS environment at database start time, and these processes continue to run within the CPU
while the database is running.
An Oracle database has disk storage. This disk storage is mapped to the database server, and a request for Oracle data ultimately translated
into an OS request to perform an I/O operation against the disk.
The OS services the request and then passes the blocks into the OS buffer cache.
In a client-server environment, there are remote clients that connect to the database server to get information. These clients depend upon the
network transport layer to ensure that the database requests are routed quickly to the server.