Lesson 3	Instance Overview
Objective	Identify the processes and memory structures that make up an Oracle instance.

Components that make up an Oracle Instance

The diagram below illustrates the processes and memory structures that make up an Oracle instance. When you move your mouse over a box, you will see a brief description of what each particular process does. The square boxes represent processes, while the circle in the center represents an area of memory that they all share. Boxes drawn in bold are processes that are mandatory, and that will always be running when a database is open. The other processes are optional, and whether or not they will exist depends on the specific mix of Oracle features that you are using.

Oracle Instance Components: 1) SGA, 2) DBW0 3) LGWR 4) SMON 5) PMON 6) RECO 7) SNP0

*SGA - System Global Area:* A memory area containing the database buffer cache, the redo log buffer, frequently used SQL statements, and other items critical to the operation of a database.

*DBW0 - Database Writer*: Writes modified blocks back to the datafiles.

*LGWR - Log Writer:* Writes the database's redo log.

SMON - System Monitor: Performs crash recovery, coalesces free space, etc.

PMON - Process Monitor: Cleans up after user processes.

RECO - Recoverer: Resolves distributed transactions.

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.

SGA - System Global Area: A memory area containing the database buffer cache, the redo log buffer, frequently used SQL statements, and other items critical to the operation of a database

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

Simultaneous processes

You will notice that several of the process names end with one or more zeros. DBW0 is one example. An Oracle instance can be configured to have several such processes running simultaneously. For example, if your database experiences a high volume of updates,, you might configure the instance to run with two database writers, which would be named DBW0 and DBW1. This generally only makes sense when you have multiple CPUs to run those processes. The procedures for displaying a list of running processes are different, and depend on whether you are using

Listing processes on a UNIX system

You can use the UNIX ps command to generate a list of running Oracle processes. The way this is generally done is to use

ps -ef

to see an extended process status listing, and then grepping for the processes of interest. For example, it's very common to run multiple instances on the same server, and if you are not sure which instances you have running, you can find out by grepping for database writers. Every instance has at least one database writer, so you are sure not to miss any Here's an example:

$ ps -ef | grep dbw0
oracle 2564   1  2 07:00:01 ?     0:05 ora_dbw0_test
oracle 2588   1  2 07:00:05 ?     0:05 ora_dbw0_DEVL
oracle 1387   1  0  Mar 25  ?     0:17 ora_dbw0_orcl
oracle 2576   1  1 07:00:03 ?     0:06 ora_dbw0_SEED
oracle 3351 3336 1 17:19:02 ttyp2 0:00 grep dbw0

Notice the process names. The first part is always “ora.” The second part identifies the specific Oracle process, dbw0, lgwr, and so on. The third part is the SID , which identifies the instance. This naming convention enables you to have multiple instances, or databases, on one computer. To see a list of all processes running for an instance, just grep for the SID. For example:

$ ps -ef | grep DEVL
  oracle  2592   1  0 07:00:05 ?  0:00 ora_smon_DEVL
  oracle  2590   1  0 07:00:05 ?  0:03 ora_lgwr_DEVL
  oracle  2588   1  0 07:00:05 ?  0:05 ora_dbw0_DEVL
  oracle  2586   1  0 07:00:05 ?  0:03 ora_pmon_DEVL
  oracle  2594   1  0 07:00:05 ?  0:00 ora_reco_DEVL
  oracle  2597   1  0 07:00:05 ?  0:03 ora_arc0_DEVL

If you want to see all the Oracle processes period, you can grep for the first part of the name: “ora.”

Windows.

When an instance is started, Oracle Database allocates a memory area and starts background processes. The memory area stores information such as the following:

Program code
Information about each connected session, even if it is not currently active
Information needed during program execution, for example, the current state of a query from which rows are being fetched
Information such as lock data that is shared and communicated among processes
Cached data, such as data blocks and redo records, that also exists on disk

Oracle Process Instance - Quiz

Before going on, click the Quiz link below to take this quiz and your knowledge of the process abbreviations.
Oracle Process Instance - Quiz