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为何而心跳-Oracle Heartbeat研究之二

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在Oracle中有一个事件叫Heartbeat，这个词在很多地方被提及，并且有着不同的含义(比如RAC中)，我们这里要讨论的是CKPT的Heartbeat机制。

作者：盖国强来源：eygle.com 2007年10月2日

在上一篇文章(为何而心跳-Oracle Heartbeat研究)中,我简单介绍了heartbeat的机制,现在我们来作点进一步的研究.

首先启动数据库到Mount状态:

[oracle@jumper bdump]$ sqlplus '/ as sysdba'

SQL*Plus: Release 9.2.0.4.0 - Production on Tue Jan 24 14:10:02 2006

Copyright (c) 1982, 2002, Oracle Corporation.  All rights reserved.

Connected to an idle instance.

SQL> startup mount;
ORACLE instance started.

Total System Global Area   97588504 bytes
Fixed Size                   451864 bytes
Variable Size              33554432 bytes
Database Buffers           62914560 bytes
Redo Buffers                 667648 bytes
Database mounted.
SQL> exit
Disconnected from Oracle9i Enterprise Edition Release 9.2.0.4.0 - Production
With the Partitioning option
JServer Release 9.2.0.4.0 - Production

在Mount状态DUMP控制文件,比较前后变化,我们发现Heartbeat每3秒都被更新一次:

[oracle@jumper udump]$ sqlplus "/ as sysdba"

SQL*Plus: Release 9.2.0.4.0 - Production on Tue Jan 24 14:10:33 2006

Copyright (c) 1982, 2002, Oracle Corporation.  All rights reserved.


Connected to:
Oracle9i Enterprise Edition Release 9.2.0.4.0 - Production
With the Partitioning option
JServer Release 9.2.0.4.0 - Production

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

Session altered.

SQL> exit
Disconnected from Oracle9i Enterprise Edition Release 9.2.0.4.0 - Production
With the Partitioning option
JServer Release 9.2.0.4.0 - Production

[oracle@jumper udump]$ sqlplus "/ as sysdba"

SQL*Plus: Release 9.2.0.4.0 - Production on Tue Jan 24 14:10:46 2006

Copyright (c) 1982, 2002, Oracle Corporation.  All rights reserved.


Connected to:
Oracle9i Enterprise Edition Release 9.2.0.4.0 - Production
With the Partitioning option
JServer Release 9.2.0.4.0 - Production

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

Session altered.

SQL> exit
Disconnected from Oracle9i Enterprise Edition Release 9.2.0.4.0 - Production
With the Partitioning option
JServer Release 9.2.0.4.0 - Production
[oracle@jumper udump]$ ls
conner_ora_31841.trc  conner_ora_31846.trc
[oracle@jumper udump]$ diff conner_ora_31841.trc conner_ora_31846.trc 
....
16c16
< *** SESSION ID:(9.5) 2006-01-24 14:10:39.076
---
> *** SESSION ID:(9.7) 2006-01-24 14:10:48.822
63c63
< heartbeat: 580556236 mount id: 3192501183
---
> heartbeat: 580556239 mount id: 3192501183

这说明Heartbeat并非在数据库Open状态下才会更新,也说明HeartBeat是用来维持实例的Mount状态检测.

Q:这里的mount id应该代表的是instance的id,每次不同,这个id来自何处呢?应该和系统有关,我没找到具体的含义.哪位知道请告知.

之所以验证这个内容是因为在Steve ( www.ixora.com.au )的网站上有这样一段话:

The checkpoint RBA is copied into the checkpoint progress record of the controlfile by the checkpoint heartbeat once every 3 seconds.

这段话在不同的数据库版本里需要被重新理解.

另外我们还可以看到,在Mount状态下,数据库通过以下两个锁定来维持Instance的变化:

SQL> select * from v$lock;

ADDR     KADDR           SID TYPE        ID1        ID2      LMODE    REQUEST      CTIME      BLOCK
-------- -------- ---------- ---- ---------- ---------- ---------- ---------- ---------- ----------
562E35F4 562E3604          3 RT            1          0          6          0       1616          0
562E34C4 562E34D4          4 XR            4          0          1          0       1619          0

其中RT锁是:Redo thread global enqueue ,为LGWR进程持有.

XR锁Oracle的解释为:

acquired for ALTER SYSTEM QUIESCE RESTRICTED command (or alter database open) in RAC mode."

此处为CKPT进程持有.

在数据库Mount状态下,我们也可以通过查询X$KCCRT来观察heartbeat的变化:

[oracle@jumper oracle]$ sqlplus '/ as sysdba'

SQL*Plus: Release 9.2.0.4.0 - Production on Wed Jan 25 00:08:06 2006

Copyright (c) 1982, 2002, Oracle Corporation.  All rights reserved.


Connected to:
Oracle9i Enterprise Edition Release 9.2.0.4.0 - Production
With the Partitioning option
JServer Release 9.2.0.4.0 - Production

SQL> select cphbt from X$KCCCP;

     CPHBT
----------
 580567934

SQL> /

     CPHBT
----------
 580567935

SQL> /

     CPHBT
----------
 580567936

SQL> select open_mode from v$database;

OPEN_MODE
----------
MOUNTED

SQL>

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