ORACLE checkpoint cnt和checkpoint scn

刚才上网搜checkpoint cnt，看到EYGLE写的一篇文章，写得不错，存起来方便以后学习。

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通过试验说明checkpoint cnt 和checkpoint scn的关系

1.在不同条件下转储控制文件

SQL> alter session set events 'immediate trace name CONTROLF level 10'; Session altered. SQL> alter tablespace system begin backup; Tablespace altered. SQL> alter session set events 'immediate trace name CONTROLF level 10'; Session altered. SQL> alter system checkpoint; System altered. SQL> alter session set events 'immediate trace name CONTROLF level 10' 2 / Session altered. SQL> alter tablespace system end backup; Tablespace altered. SQL> alter session set events 'immediate trace name CONTROLF level 10'; Session altered.

notes:

alter session set events 'immediate trace name CONTROLF level 10';

用于转储控制文件.

 

2.获得以下跟踪文件信息(仅研究system表空间记录,请注意红色部分):

a.正常情况下转储控制文件

*************************************************************************** DATA FILE RECORDS *************************************************************************** (blkno = 0x6, size = 180, max = 100, in-use = 24, last-recid= 574) DATA FILE #1: (name #4) /opt/oracle/oradata/hsjf/system01.dbf creation size=32000 block size=8192 status=0xe head=4 tail=4 dup=1 tablespace 0, index=1 krfil=1 prev_file=0 unrecoverable scn: 0x0000.00000000 04/23/2004 01:20:52 Checkpoint cnt:1567 scn: 0x0000.0148181c 06/22/2004 18:58:46 Stop scn: 0xffff.ffffffff 06/22/2004 18:58:05 Creation Checkpointed at scn: 0x0000.000000ae 07/16/2003 03:40:10 thread:1 rba:(0x1.3.10) enabled threads: 01000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 Offline scn: 0x0000.013b46fd prev_range: 0 Online Checkpointed at scn: 0x0000.013b46fe 05/28/2004 23:37:17 thread:1 rba:(0x1.2.0) enabled threads: 01000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 Hot Backup end marker scn: 0x0000.00000000 aux_file is NOT DEFINED

b.执行Begin backup以后的

我们注意到Checkpoint cnt增加了1，此处触发了一次表空间检查点.

*************************************************************************** DATA FILE RECORDS *************************************************************************** (blkno = 0x6, size = 180, max = 100, in-use = 24, last-recid= 574) DATA FILE #1: (name #4) /opt/oracle/oradata/hsjf/system01.dbf creation size=32000 block size=8192 status=0xe head=4 tail=4 dup=1 tablespace 0, index=1 krfil=1 prev_file=0 unrecoverable scn: 0x0000.00000000 04/23/2004 01:20:52 Checkpoint cnt:1568 scn: 0x0000.01481939 06/22/2004 19:02:22 Stop scn: 0xffff.ffffffff 06/22/2004 18:58:05 Creation Checkpointed at scn: 0x0000.000000ae 07/16/2003 03:40:10 thread:1 rba:(0x1.3.10) enabled threads: 01000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 Offline scn: 0x0000.013b46fd prev_range: 0 Online Checkpointed at scn: 0x0000.013b46fe 05/28/2004 23:37:17 thread:1 rba:(0x1.2.0) enabled threads: 01000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 Hot Backup end marker scn: 0x0000.00000000 aux_file is NOT DEFINED

c.执行手工检查点

我们注意到,此时Checkpoint cnt增加，但是scn不再改变

*************************************************************************** DATA FILE RECORDS *************************************************************************** (blkno = 0x6, size = 180, max = 100, in-use = 24, last-recid= 574) DATA FILE #1: (name #4) /opt/oracle/oradata/hsjf/system01.dbf creation size=32000 block size=8192 status=0xe head=4 tail=4 dup=1 tablespace 0, index=1 krfil=1 prev_file=0 unrecoverable scn: 0x0000.00000000 04/23/2004 01:20:52 Checkpoint cnt:1569 scn: 0x0000.01481939 06/22/2004 19:02:22 Stop scn: 0xffff.ffffffff 06/22/2004 18:58:05 Creation Checkpointed at scn: 0x0000.000000ae 07/16/2003 03:40:10 thread:1 rba:(0x1.3.10) enabled threads: 01000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 Offline scn: 0x0000.013b46fd prev_range: 0 Online Checkpointed at scn: 0x0000.013b46fe 05/28/2004 23:37:17 thread:1 rba:(0x1.2.0) enabled threads: 01000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 Hot Backup end marker scn: 0x0000.00000000 aux_file is NOT DEFINED

d.End backup后的情况

此时数据文件头的冻结被取消,scn开始变化

*************************************************************************** DATA FILE RECORDS *************************************************************************** (blkno = 0x6, size = 180, max = 100, in-use = 24, last-recid= 574) DATA FILE #1: (name #4) /opt/oracle/oradata/hsjf/system01.dbf creation size=32000 block size=8192 status=0xe head=4 tail=4 dup=1 tablespace 0, index=1 krfil=1 prev_file=0 unrecoverable scn: 0x0000.00000000 04/23/2004 01:20:52 Checkpoint cnt:1570 scn: 0x0000.01481941 06/22/2004 19:02:39 Stop scn: 0xffff.ffffffff 06/22/2004 18:58:05 Creation Checkpointed at scn: 0x0000.000000ae 07/16/2003 03:40:10 thread:1 rba:(0x1.3.10) enabled threads: 01000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 Offline scn: 0x0000.013b46fd prev_range: 0 Online Checkpointed at scn: 0x0000.013b46fe 05/28/2004 23:37:17 thread:1 rba:(0x1.2.0) enabled threads: 01000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000 Hot Backup end marker scn: 0x0000.00000000 aux_file is NOT DEFINED

Checkpoint cnt用于保证在正常操作中使用的数据文件是当前版本
在恢复时防止恢复数据文件的错误版本.Checkpoint cnt是一直递增的,即使表空间处于热备份模式.

由于表空间的创建时间不尽相同,所以不同表空间/数据文件的Checkpoint cnt通常是不同的.

我们知道:

在数据库open的过程中,Oracle要进行两次检查.

第一次检查数据文件头中的Checkpoint cnt是否与对应控制文件中的Checkpoint cnt一致.
如果相等,进行第二次检查.

第二次检查数据文件头的开始SCN和对应控制文件中的结束SCN是否一致
如果结束SCN等于开始SCN,则不需要对那个文件进行恢复.

对每个数据文件都完成检查后,打开数据库.同时将每个数据文件的结束SCN设置为无穷大.