做了个小实验，看看将数据库改为归档模式的情况下，后台进程的启动情况。

SQL> select distinct name,description from v$bgprocess where paddr<>'00'

NAME DESCRIPTION
----- ----------------------------------------------------------------
CKPT checkpoint
QMNC AQ Coordinator
MMNL Manageability Monitor Process 2
PSP0 process spawner 0
DBW0 db writer process 0
SMON System Monitor Process
RECO distributed recovery
PMON process cleanup
CJQ0 Job Queue Coordinator
LGWR Redo etc.
MMON Manageability Monitor Process

NAME DESCRIPTION
----- ----------------------------------------------------------------
MMAN Memory Manager

12 rows selected.

--非归档模式下启动了12个进程

SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup mount
ORACLE instance started.

Total System Global Area 167772160 bytes
Fixed Size                  1247876 bytes
Variable Size              71304572 bytes
Database Buffers           88080384 bytes
Redo Buffers                7139328 bytes
Database mounted.
SQL> alter database archivelog;

Database altered.

SQL> alter database open;

Database altered.

SQL> select distinct name,description from v$bgprocess where paddr<>'0

NAME DESCRIPTION
----- ----------------------------------------------------------------
ARC2 Archival Process 2
CKPT checkpoint
QMNC AQ Coordinator
MMNL Manageability Monitor Process 2
PSP0 process spawner 0
DBW0 db writer process 0
SMON System Monitor Process
RECO distributed recovery
PMON process cleanup
CJQ0 Job Queue Coordinator
LGWR Redo etc.

NAME DESCRIPTION
----- ----------------------------------------------------------------
MMON Manageability Monitor Process
MMAN Memory Manager
ARC0 Archival Process 0
ARC1 Archival Process 1

15 rows selected.

--改为归档模式后，后台启动了15个进程，多出的三个进程是ARC0 ，ARC1，ARC2

SQL> shutdown immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup mount
ORACLE instance started.

Total System Global Area 167772160 bytes
Fixed Size                  1247876 bytes
Variable Size              71304572 bytes
Database Buffers           88080384 bytes
Redo Buffers                7139328 bytes
Database mounted.
SQL> alter database noarchivelog
2 ;

Database altered.

SQL> alter database open;

Database altered.

SQL> select distinct name,description from v$bgprocess where paddr<>'00';

NAME DESCRIPTION
----- ----------------------------------------------------------------
CKPT checkpoint
QMNC AQ Coordinator
MMNL Manageability Monitor Process 2
PSP0 process spawner 0
DBW0 db writer process 0
SMON System Monitor Process
RECO distributed recovery
PMON process cleanup
CJQ0 Job Queue Coordinator
LGWR Redo etc.
MMON Manageability Monitor Process

NAME DESCRIPTION
----- ----------------------------------------------------------------
MMAN Memory Manager

12 rows selected.
--再切换回非归档模式，后台进程又变为12个。

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切换归档模式后后台进程的情况

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