数据库启动过程图及SCN讲解

数据库启动过程图

日志文件中

• FIRST SCN：redo log file中第一条日志的SCN

• NEXT SCN：redo log file中最后一条日志的SCN(即下一个redo log file的第一条日志的SCN)

通常,只有当前的重做日志文件组写满后才发生日志切换,但是可以通过设置参数ARCHIVE_LOG_TARGET控制日志切换的时间间隔,在必要时也可以采用手工强制进行日志切换.

一组redo log file写满后，会自动切换到下一组redo log file。上一组redo log的High SCN就是下一组redo log的Low SCN,且对于Current日志文件的High SCN为无穷大(FFFFFFFF)。

在open数据库时,Oracle通过控制文件进行了以下验证：

1. 检查数据文件头部所记录的Start SCN 和控制文件中所记录的System Checkpoint SCN 是否一致,若不同则需要进行介质恢复

2. 检查数据文件头部所记录的Start SCN 和控制文件中记录的Stop SCN是否也一致,若不同则需要进行实例恢复.

如果两个都一致了,说明所有已被修改的数据块已经写入到了数据文件中,才可以正常open,

当数据库open并正常运行期间,系统SCN、文件SCN和数据文件头部的开始SCN都是一致的,且(大于或)等于ACTIVE/CURRENT日志文件的最小FIRST SCN,但文件结束SCN为NULL(无穷大);

当数据库正常关闭时,Oracle通过完全检查点将buffer cache中的所有缓存写到磁盘上,同时根据关闭数据库的时间点更新控制文件中的系统SCN、文件SCN、结束SCN和数据文件头部中的开始SCN,且SCN都是一致的,且LRBA指针指向on disk RBA,否则需要前滚;

当数据库非正常关闭(崩溃/掉电)后启动实例时,Oracle将检测到控制文件中的系统SCN、文件SCN和数据文件头部的开始SCN都是一致的,但是结束SCN为NULL,则在需要参与实例崩溃恢复的redo log file中根据控制文件中记录的LRBA地址(前滚起点)和on disk RBA(前滚终点)地址找出相应的日志项进行实例崩溃恢复,最终才可将数据库open.

• 回滚阶段(回滚靠undo,又叫事务恢复transaction recoery,即负责回退实例崩溃前没有提交的事务)

正常关闭数据库时：

系统SCN、文件SCN、结束SCN和数据文件头部中的开始SCN都是相等的，且(大于或)等于ACTIVE/CURRENT日志文件中的最小FIRST SCN

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SQL> shutdown immediate

Database closed.

Database dismounted.

ORACLE instance shut down.

SQL> startup mount

ORACLE instance started.

Total System Global Area  459304960 bytes

Fixed Size                 2214336 bytes

Variable Size            289408576 bytes

Database Buffers          159383552 bytes

Redo Buffers                8298496 bytes

Databasemounted.

SQL>  selectcheckpoint_change# fromv$database;

CHECKPOINT_CHANGE#

------------------

           1822573

SQL>  selectname,checkpoint_change#,last_change#from v$datafile;

NAME                                         CHECKPOINT_CHANGE# LAST_CHANGE#

--------------------------------------------- ------------------ ------------

+DATA/orcl/datafile/system.256.817343229                 1822573      1822573

+DATA/orcl/datafile/sysaux.257.817343231                 1822573      1822573

+DATA/orcl/datafile/undotbs1.258.817343231               1822573      1822573

+DATA/orcl/datafile/users.259.817343231                  1822573      1822573

+DATA/orcl/datafile/example.265.817343543                1822573      1822573

SQL> selectname,checkpoint_change# from v$datafile_header;

NAME                                         CHECKPOINT_CHANGE#

--------------------------------------------- ------------------

+DATA/orcl/datafile/system.256.817343229                 1822573

+DATA/orcl/datafile/sysaux.257.817343231                 1822573

+DATA/orcl/datafile/undotbs1.258.817343231               1822573

+DATA/orcl/datafile/users.259.817343231                  1822573

+DATA/orcl/datafile/example.265.817343543                1822573

SQL> selectgroup#,sequence#,status,first_change#,next_change#from v$log;

    GROUP# SEQUENCE# STATUS           FIRST_CHANGE#       NEXT_CHANGE#

---------- ---------- ---------------- ------------- ------------------

         1         37CURRENT                1822207    281474976710655

         3         36 INACTIVE               1808596            1822207

         2         35 INACTIVE               1770739            1808596

正常open数据库时：

文件结束SCN为NULL(无穷大)

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SQL> alterdatabase open;

Database altered.

SQL> selectname,checkpoint_change#,last_change#from v$datafile;

NAME                                         CHECKPOINT_CHANGE# LAST_CHANGE#

--------------------------------------------- ------------------ ------------

+DATA/orcl/datafile/system.256.817343229                 1822576

+DATA/orcl/datafile/sysaux.257.817343231                 1822576

+DATA/orcl/datafile/undotbs1.258.817343231               1822576

+DATA/orcl/datafile/users.259.817343231                  1822576

+DATA/orcl/datafile/example.265.817343543                1822576

异常关机（实例崩溃）时：

文件结束SCN仍为NULL(无穷大)

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SQL> shutdown abort

ORACLE instance shut down.

SQL> startup mount

ORACLE instance started.

Total System Global Area  459304960 bytes

Fixed Size                 2214336 bytes

Variable Size            289408576 bytes

Database Buffers          159383552 bytes

Redo Buffers                8298496 bytes

Database mounted.

SQL> selectname,checkpoint_change#,last_change#from v$datafile;

NAME                                         CHECKPOINT_CHANGE# LAST_CHANGE#

--------------------------------------------- ------------------ ------------

+DATA/orcl/datafile/system.256.817343229                 1822576

+DATA/orcl/datafile/sysaux.257.817343231                 1822576

+DATA/orcl/datafile/undotbs1.258.817343231               1822576

+DATA/orcl/datafile/users.259.817343231                  1822576

+DATA/orcl/datafile/example.265.817343543                1822576

启动实例将进行实例恢复：

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SQL> alterdatabase open;

Database altered.

$ tailf /u01/app/oracle/diag/rdbms/orcl/orcl/trace/alert_orcl.log

Sun Jul 07 00:10:07 2013

alter database open

Beginning crash recoveryof 1 threads

 parallel recovery startedwith 3 processes

Started redo scan

Completed redo scan

 read192 KB redo, 87 data blocks need recovery

Started redo applicationat

 Thread 1: logseq 37, block 533

Recovery ofOnline Redo Log: Thread 1 Group1 Seq 37 Reading mem 0

  Mem# 0: +DATA/orcl/onlinelog/group_1.261.817343457

  Mem# 1: +FRA/orcl/onlinelog/group_1.257.817343463

Completed redo applicationof 0.15MB

Completed crash recoveryat

 Thread 1: logseq 37, block 918, scn 1843004

 87 data blocksread, 87 data blocks written, 192 redo k-bytesread

Sun Jul 07 00:10:13 2013

Thread 1 advancedto log sequence38 (thread open)

Thread 1 opened at log sequence38

  Currentlog# 2 seq# 38 mem# 0: +DATA/orcl/onlinelog/group_2.262.817343467

  Currentlog# 2 seq# 38 mem# 1: +FRA/orcl/onlinelog/group_2.258.817343473

Successful open of redo thread 1

Sun Jul 07 00:10:14 2013

SMON: enabling cache recovery

Successfully onlined Undo Tablespace 2.

Verifying file header compatibilityfor 11g tablespace encryption..

Verifying 11g file header compatibilityfor tablespace encryption completed

SMON: enabling tx recovery

DatabaseCharacterset is AL32UTF8

No Resource Manager plan active

Sun Jul 07 00:10:17 2013

replication_dependency_tracking turnedoff (noasync multimaster replication found)

Starting background process QMNC

Sun Jul 07 00:10:21 2013

QMNC started with pid=28, OS id=7140

Sun Jul 07 00:10:31 2013

Completed: alter database open