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The content of the architecture belongs to the category of OCP and is not required in OCA

The architecture includes: database (data file, parameter file, password file, log file, control file, archive log), instance (memory structure, background process)

Memory: including SGA (system global area) and PGA (program global area). In general, 80% of the memory allocated by Oracle services is for SGA and only 20% is for PGA; SGA is allocated when the instance is started and PGA is allocated when the server process starts

Background processes: including PMON, SMON, DBWR, LGWR, CKPT, etc.

SGA: a large memory area shared by multiple background processes; includes shared pools, data buffers, log buffers, Large pools, Stream pools, JAVA pools

Shared pool: a memory area that parses, compiles (generates execution plans) and executes SQL and PL/SQL. If you want to share the compiled execution plan, the SQL or PL/SQL statements must be exactly the same. The size of the shared pool will affect database performance.

Data buffer: used to store data read from disk data files and shared by all users; when updating, the corresponding rows are read into the data buffer for modification and then written to disk; the same is true when deleting operations; when inserting operations, it is also inserted into the data buffer and written to the hard disk at some point in the future; its size has an impact on database read performance.

Log buffer: add, delete and modify operations are recorded in the log buffer and written to the disk log when certain conditions are met

Large pool: the memory space allocated for large background process operations, such as backup recovery, large IO operations, parallel queries, etc.

Stream pool: memory space allocated for Stream applications; part of advanced replication

JAVA pool: memory space allocated for JAVA applications

PGA: contains the data and control information required by a single server process. PGA is exclusive.

Several important background processes:

PMON: process monitoring process: clean up failed processes, release resources used by failed processes, and release all pending locks in the face

SMON: system monitoring process: restore the instance when you reopen the database after the instance fails

DBWR: data writing process: used to manage data buffers

LGWR: log writing process: writes the contents of the log buffer to the log file when the condition is met

CKPT: in order to prevent the instance from crashing, it is complicated to restore the instance as soon as possible.

How to view background processes select * from v$process

Important features of PGA: automatic PGA management is introduced from 9i, which can specify the total size of PGA, which is allocated by Oracle, and how much PGA each server process needs.

Important feature of SGA: introduction of automatic shared memory management in 10g

Introduce a new feature in 11G: automatic memory management, which can flexibly resize SGA, PGA, and corresponding internal parts.

Oracle database: contains logical structure, physical structure (including a set of files in the database)

Files are divided into two categories: main physical files and non-main physical files. The main physical files include data files, log files and control files. Once the main files are lost or damaged, the service will not operate normally; non-main physical files include parameter files, password files, backup files, archive logs, etc., lost or damaged does not affect the normal operation of the database.

Logical structure: analyzes the composition of the database from a logical point of view, including database-tablespaces (one or more data files)-segments-regions-data blocks

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