Oracle7.3.4 Checkpoint行爲研究

在不同的版本中,Oracle的檢查點策略一直在不斷的調整優化,而從Oracle7之後,Oracle不在文檔中透漏更多的信息給我們。

關於檢查點的描述,很多文檔上大都是簡單的一句話,通常是:

When a checkpoint occurs, Oracle must update the headers of all datafiles to record the details of the checkpoint. This is done by the CKPT process. The CKPT process does not write blocks to disk; DBWn always performs that work.

這段描述實際上過於簡略了,在不同的版本中,Oracle的檢查點策略一直在不斷的調整優化,而從Oracle7之後,Oracle不在文檔中透漏更多的信息給我們.

在Oracle7中,通過跟蹤CKPT進程我們可以看到,當我們手工執行檢查點時:

C:orantBIN>SVRMGR23.EXE Oracle Server Manager Release 2.3.4.0.0 - Production Copyright (c) Oracle Corporation 1994, 1995. All rights reserved. Oracle7 Workgroup Server Release 7.3.4.0.0 - Production
With the distributed option
PL/SQL Release 2.3.4.0.0 - Production SVRMGR> connect internal
Connected to an idle instance.
SVRMGR> startup
Oracle instance started.
Total System Global Area 99967716 bytes
Fixed Size 35760 bytes
Variable Size 58808116 bytes
Database Buffers 40960000 bytes
Redo Buffers 163840 bytes
Database mounted.
Database opened.
SVRMGR> alter system checkpoint;
Statement processed.

Oracle後臺CKPT進程如下動作:

WAIT #0: nam='control file sequential read' ela= 0 p1=1 p2=1 p3=1
WAIT #0: nam='control file sequential read' ela= 0 p1=0 p2=4 p3=1
WAIT #0: nam='control file parallel write' ela= 0 p1=2 p2=2 p3=2
WAIT #0: nam='control file parallel write' ela= 0 p1=2 p2=2 p3=2
WAIT #0: nam='control file sequential read' ela= 0 p1=0 p2=5 p3=1
WAIT #0: nam='db file sequential read' ela= 0 p1=1 p2=1 p3=1
WAIT #0: nam='db file single write' ela= 0 p1=1 p2=1 p3=1
WAIT #0: nam='control file parallel write' ela= 0 p1=2 p2=2 p3=2
WAIT #0: nam='control file sequential read' ela= 0 p1=0 p2=6 p3=1
WAIT #0: nam='control file sequential read' ela= 0 p1=0 p2=8 p3=1
WAIT #0: nam='db file sequential read' ela= 0 p1=2 p2=1 p3=1
WAIT #0: nam='db file single write' ela= 0 p1=2 p2=1 p3=1
WAIT #0: nam='control file parallel write' ela= 0 p1=2 p2=2 p3=2
WAIT #0: nam='control file parallel write' ela= 0 p1=2 p2=2 p3=2
WAIT #0: nam='db file sequential read' ela= 0 p1=3 p2=1 p3=1
WAIT #0: nam='db file single write' ela= 0 p1=3 p2=1 p3=1
WAIT #0: nam='control file parallel write' ela= 0 p1=2 p2=2 p3=2
WAIT #0: nam='db file sequential read' ela= 0 p1=4 p2=1 p3=1
WAIT #0: nam='db file single write' ela= 0 p1=4 p2=1 p3=1
WAIT #0: nam='control file parallel write' ela= 0 p1=2 p2=2 p3=2
WAIT #0: nam='control file parallel write' ela= 0 p1=2 p2=2 p3=2
WAIT #0: nam='rdbms ipc message' ela= 0 p1=300 p2=0 p3=0

注意,這裏的等待事件 "db file single write" 既是對於數據文件頭的更新,以刷新數據文件記錄的檢查點信息.

同樣在Oracle7中,當發生log switch之後,觸發檢查點,CKPT進程需要更新數據文件頭及控制文件:

SVRMGR> select * from v$version;
BANNER
----------------------------------------------------------
Oracle7 Workgroup Server Release 7.3.4.0.0 - Production
PL/SQL Release 2.3.4.0.0 - Production
CORE Version 3.5.4.0.0 - Production
TNS for 32-bit Windows: Version 2.3.4.0.0 - Production
NLSRTL Version 3.2.4.0.0 - Production
5 rows selected.
SVRMGR> alter system switch logfile;
Statement processed.
SVRMGR>

我們看CKPT進程的動作:

WAIT #0: nam='control file sequential read' ela= 0 p1=0 p2=1 p3=1
WAIT #0: nam='control file sequential read' ela= 0 p1=1 p2=1 p3=1
WAIT #0: nam='control file sequential read' ela= 0 p1=0 p2=3 p3=1
WAIT #0: nam='rdbms ipc message' ela= 0 p1=202 p2=0 p3=0
WAIT #0: nam='control file sequential read' ela= 0 p1=0 p2=1 p3=1
WAIT #0: nam='control file sequential read' ela= 0 p1=1 p2=1 p3=1
WAIT #0: nam='control file sequential read' ela= 0 p1=0 p2=3 p3=1
WAIT #0: nam='control file parallel write' ela= 0 p1=2 p2=2 p3=2
WAIT #0: nam='control file parallel write' ela= 0 p1=2 p2=2 p3=2
WAIT #0: nam='control file sequential read' ela= 0 p1=0 p2=6 p3=1
WAIT #0: nam='db file sequential read' ela= 0 p1=1 p2=1 p3=1
WAIT #0: nam='db file single write' ela= 0 p1=1 p2=1 p3=1
WAIT #0: nam='control file parallel write' ela= 0 p1=2 p2=2 p3=2
WAIT #0: nam='control file sequential read' ela= 0 p1=0 p2=5 p3=1
WAIT #0: nam='control file sequential read' ela= 0 p1=0 p2=7 p3=1
WAIT #0: nam='db file sequential read' ela= 0 p1=2 p2=1 p3=1
WAIT #0: nam='db file single write' ela= 0 p1=2 p2=1 p3=1
WAIT #0: nam='control file parallel write' ela= 0 p1=2 p2=2 p3=2
WAIT #0: nam='control file parallel write' ela= 0 p1=2 p2=2 p3=2
WAIT #0: nam='db file sequential read' ela= 0 p1=3 p2=1 p3=1
WAIT #0: nam='db file single write' ela= 0 p1=3 p2=1 p3=1
WAIT #0: nam='control file parallel write' ela= 0 p1=2 p2=2 p3=2
WAIT #0: nam='db file sequential read' ela= 0 p1=4 p2=1 p3=1
WAIT #0: nam='db file single write' ela= 0 p1=4 p2=1 p3=1
WAIT #0: nam='control file parallel write' ela= 0 p1=2 p2=2 p3=2
WAIT #0: nam='control file parallel write' ela= 0 p1=2 p2=2 p3=2
WAIT #0: nam='rdbms ipc message' ela= 0 p1=300 p2=0 p3=0

在Oracle7中,logfile switch 和 alter system checkpoint都會觸發complete checkpoint, 此時DBWR會把Dirty Buffer寫出到數據文件. 從DBWR的跟蹤我們可以輕易的看出.

WAIT #0: nam='rdbms ipc message' ela= 0 p1=300 p2=0 p3=0
WAIT #0: nam='db file parallel write' ela= 0 p1=2 p2=3 p3=3
WAIT #0: nam='db file parallel write' ela= 0 p1=2 p2=3 p3=3
WAIT #0: nam='db file parallel write' ela= 0 p1=1 p2=2 p3=2
WAIT #0: nam='rdbms ipc message' ela= 0 p1=218 p2=0 p3=0

而從Oracle8i開始引入增量檢查點之後,Oracle的檢查點行爲就發生了改變(待續...).