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数据库启动过程图
日志文件中
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FIRST SCN：redo log file中第一条日志的SCN
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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：
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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
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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.
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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
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 | 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 Database mounted. SQL> select checkpoint_change# from v$ database ; CHECKPOINT_CHANGE# ------------------ 1822573 SQL> select name ,checkpoint_change#,last_change# from v$datafile; NAME CHECKPOINT_CHANGE# LAST_CHANGE# --------------------------------------------- ------------------ ------------ +DATA/orcl/datafile/system.256.817343229 1822573 1822573 +DATA/orcl/datafile/sysaux.257.817343231 1822573 1822573 +DATA/orcl/datafile/undotbs1.258.817343231 1822573 1822573 +DATA/orcl/datafile/users.259.817343231 1822573 1822573 +DATA/orcl/datafile/example.265.817343543 1822573 1822573 SQL> select name ,checkpoint_change# from v$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> select group #, sequence #,status,first_change#,next_change# from v$log; GROUP # SEQUENCE # STATUS FIRST_CHANGE# NEXT_CHANGE# ---------- ---------- ---------------- ------------- ------------------ 1 37 CURRENT 1822207 281474976710655 3 36 INACTIVE 1808596 1822207 2 35 INACTIVE 1770739 1808596 |
正常open数据库时：
文件结束SCN为NULL(无穷大)
| 1 2 3 4 5 6 7 8 9 10 | SQL> alter database open ; Database altered. SQL> select name ,checkpoint_change#,last_change# from v$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)
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 | 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> select name ,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：
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 | SQL> alter database 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 recovery of 1 threads parallel recovery started with 3 processes Started redo scan Completed redo scan read 192 KB redo, 87 data blocks need recovery Started redo application at Thread 1: logseq 37, block 533 Recovery of Online Redo Log: Thread 1 Group 1 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 application of 0.15MB Completed crash recovery at Thread 1: logseq 37, block 918, scn 1843004 87 data blocks read , 87 data blocks written, 192 redo k-bytes read Sun Jul 07 00:10:13 2013 Thread 1 advanced to log sequence 38 (thread open ) Thread 1 opened at log sequence 38 Current log# 2 seq# 38 mem# 0: +DATA/orcl/onlinelog/group_2.262.817343467 Current log# 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 compatibility for 11g tablespace encryption.. Verifying 11g file header compatibility for tablespace encryption completed SMON: enabling tx recovery Database Characterset is AL32UTF8 No Resource Manager plan active Sun Jul 07 00:10:17 2013 replication_dependency_tracking turned off ( no async 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 |
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