oracle體系結構

Component

Description

Database buffer cache

Before data stored in the database can be queried or modified, it must be read from a disk and stored

in the buffer cache. All user processes connected to the database share access to the buffer cache.

 For optimal performance, the buffer cache should be large enough to avoid frequent disk I/O

operations.

Shared pool            

The shared pool caches information that is shared among users:

·         SQL statements that can be reused

·         Information from the data dictionary such as user account data, table and index descriptions,

·          and privileges

·         Stored procedures, which are executable code that is stored in the database

Redo log buffer

This buffer improves performance by caching redo information until it can be written to the physical

online redo log files stored on disk. Redo information and online redo log files are discussed in

 .

Large pool

This optional area is used to buffer large I/O requests for various server processes.

Java pool

The Java pool is an area of memory that is used for all session-specific Java code and data within the

 Java Virtual Machine (JVM).

Streams pool

The Streams pool is an area of memory that is used by the Oracle Streams feature.

 For more information about Oracle Streams, see .

Result cache

The result cache buffers query results. If a query is run for which the results are stored in the result

cache, then the database returns the query results from the result cache instead of rerunning the query.

 This SGA component speeds the execution of frequently run queries.

Background Process

Description

Database writer (DBWn)

The database writer writes modified blocks from the database buffer cache to the

files on a disk. Oracle Database allows a maximum of 36 database writer processes.

Log writer (LGWR)

The log writer process writes redo log entries to disk. Redo log entries are generated

in the redo log buffer of the System Global Area (SGA) and the log writer process writes

the redo log entries sequentially into an online redo log file.

Checkpoint (CKPT)

At specific times, all modified database buffers in the SGA are written to the data files

by a database writer process (DBWn). This event is called a checkpoint.

The checkpoint process signals DBWn, updates the data files and

control files of the database, and records the time of this update.

System monitor (SMON)

The system monitor performs instance recovery when a failed instance is restarted.

Process monitor (PMON)

The process monitor performs a recovery when a user process fails.

 It cleans up the cache and frees resources that the failed process was using.

Archiver (ARCn)

Archiver processes copy the online redo log files to archival storage

when the log files are full or a log switch occurs.

The database must be in archive log mode to run archive processes.

 For more information, see .

Manageability monitor (MMON)

This process performs various management-related background tasks, for example:

·         Issuing alerts whenever a given metric violates its threshold value

·         Taking snapshots by spawning additional processes

·         Capturing statistical values for SQL objects that have been recently modified

Job Queue Processes (CJQ0 and Jnnn)

Job queue processes run user jobs, often in batch mode.

 A job is a user-defined task scheduled to run one or more times.