Oracle 基础

Table 5-1 SGA Components

Component	Description
Buffer Cache	Before any data stored in the database can be queried or modified, it must be read from disk and stored in memory in the buffer cache. All user processes connected to the database share access to the buffer cache.
Shared Pool	The shared pool caches information that can be 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 is executable code that is stored in the database
Redo Log Buffer	This buffer improves performance by caching redo information which is used for instance recovery until it can be written once at a more opportune time to the physical redo log files stored on disk. Redo information and redo log files are discussed in "Online Redo Log Files".
Large Pool	This optional area is used to buffer large I/O requests for various server processes.
Java Pool	The Java pool memory is used for all session-specific Java code and data within the Java Virtual Machine (JVM)
Streams Pool	The Streams pool is used by the Oracle Streams product.

Table 5-2 Oracle Background Processes

Background Process	Description
Database Writer (DBWn)	The database writer writes modified blocks from the database buffer cache to the files on disk. Oracle allows a maximum of 20 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 SGA and the log writer process writes the redo log entries sequentially into an online redo log file.
Checkpoint	At specific times, all modified database buffers in the SGA are written to the datafiles by a database writer process (DBWn). This event is called a checkpoint. The checkpoint process signals DBWn, updates all of the datafiles and control files of the database, and records the time of this update.
System Monitor (SMON)	The system monitor performs crash recovery when a failed instance starts up again.
Process Monitor (PMON)	The process monitor performs 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 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 process. For more information, see Chapter 9, "Performing Backup and Recovery".

Table 6-1 Tablespaces and Descriptions

Tablespace	Description
EXAMPLE	This tablespace contains the sample schemas that Oracle includes with the database. The sample schemas provide a common platform for examples. Oracle documentation and educational materials contain examples based on the sample schemas.
SYSAUX	This is an auxiliary tablespace to the SYSTEM tablespace. Some components and products that used the SYSTEM tablespace or their own tablespaces prior to Oracle Database 10g now use the SYSAUX tablespace. Using SYSAUX reduces the load on the SYSTEM tablespace and reduces maintenance because there are fewer tablespaces to monitor and maintain. Every Oracle Database 10g or higher level database must have a SYSAUX tablespace. Components that use SYSAUX as their default tablespace during installation include Automatic Workload Repository, Oracle Streams, Oracle Text, and Enterprise Manager Repository. For more information, see the Oracle Database Administrator's Guide.
SYSTEM	This tablespace is always created at database creation. Oracle uses it to manage the database. It contains the data dictionary, which is the central set of tables and views used as a read-only reference for a particular database. It also contains various tables and views that contain administrative information about the database. These are all contained in the SYS schema, and can only be accessed by user SYS or other administrative users with the required privilege.
TEMP	This tablespace stores temporary data generated when processing SQL statements. For example, this tablespace would be used for query sorting. Every database should have a temporary tablespace that is assigned to users as their temporary tablespace. In the preconfigured database, the TEMP tablespace is specified as the default temporary tablespace. This means that if no temporary tablespace is specified when a user account is created, then Oracle assigns this tablespace to the user.
UNDOTBS1	This is the undo tablespace used by the database server to store undo information. See "Managing Undo for Your Database" to understand how an Oracle database uses the undo tablespace. Every database must have an undo tablespace.
USERS	This tablespace is used to store permanent user objects and data. Like the TEMP tablespace, every database should have a tablespace for permanent user data that is assigned to users. Otherwise, user objects will be created in the SYSTEM tablespace, which is not good practice. In the preconfigured database, USERS is assigned the default tablespace, and space for all objects created by non-system users comes from this tablespace. For system users, the default permanent tablespace remains SYSTEM.