Component

Single Instance Environment

RAC Environment

SGA

Instance has its own SGA

Each instance has its own SGA

Background processes

Instance has its own set of
background processes

Each instance has its own set
of background processes

Datafiles

Accessed by only one instance

Shared by all instances
(shared storage)

Control Files

Accessed by only one instance

Shared by all instances
(shared storage)

Online Redo Logfile

Dedicated for write/read to
only one instance

Only one instance can write
but other instances can read during recovery and archiving. If an instance is
shutdown, log switches by other instances can force the idle instance redo logs
to be archived

Archived Redo Logfile

Dedicated to the instance

Private to the instance but
other instances will need access to all required archive logs during media
recovery

Flash Recovery Log

Accessed by only one instance

Shared by all instances
(shared storage)

Alert Log and Trace Files

Dedicated to the instance

Private to each instance,
other instances never read or write to those files.

ORACLE_HOME

Multiple instances on the same
server accessing different databases ca use the same executable files

Same as single instance plus
can be placed on shared file system allowing a common ORACLE_HOM E for all
instances in a RAC environment.