关于checkpoint cnt和checkpoint scn

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.