前幾天有位朋友在留言板上提了這樣一個問題：

Fri May 25 20:46:06 2007　　//自動備份controlfile

Starting control autobackup

Control autobackup written to DISK device

handle '/ora_rman_backup/crtl_backup/c-4205638757-20070525-00'

Fri May 25 20:46:11 2007　　 //發生日志切換

Thread 1 advanced to log sequence 535

Current log# 1 seq# 535 mem# 0: /dev/udpay/lv_redo11_256m

Current log# 1 seq# 535 mem# 1: /dev/udpay/lv_redo12_256m

Fri May 25 20:46:11 2007　　 //同時啟動歸檔

ARCH: Evaluating archive　 log 6 thread 1 sequence 534

ARCH: Beginning to archive log 6 thread 1 sequence 534

Creating archive destination LOG_ARCHIVE_DEST_1: '/archivelog/arch/1_534.dbf'

Fri May 25 20:46:12 2007　　 //一秒后另一進程啟動，失敗

ARC0: Evaluating archive　 log 6 thread 1 sequence 534

ARC0: Unable to archive log 6 thread 1 sequence 534

Log actively being archived by another process

Fri May 25 20:46:15 2007　　 //3秒后成功歸檔

ARCH: Completed archiving　log 6 thread 1 sequence 534

Fri May 25 20:46:18 2007　　 //3秒后日志切換

Thread 1 advanced to log sequence 536

Current log# 2 seq# 536 mem# 0: /dev/udpay/lv_redo21_256m

以上日志是備份過程中產生的，備份腳本如下：

$ORACLE_HOME/bin/rman

connect target sys/sys@udpay;

CONFIGURE CONTROLFILE AUTOBACKUP FORMAT FOR DEVICE TYPE DISK TO '/ora_rman_backup/crtl_backup/%F';

CONFIGURE CONTROLFILE AUTOBACKUP ON;

CROSSCHECK ARCHIVELOG ALL;

run{

ALLOCATE CHANNEL ch1 DEVICE TYPE DISK;

backup full database　format 'D:/%T_%d' include current controlfile;

sql 'alter system archive log current';

backup　filesperset 3 format 'D:/arch%u_%s_%p' archivelog all;

DELETE NOPROMPT ARCHIVELOG UNTIL TIME 'SYSDATE-2';

release channel ch1;

}

!!

這位朋友提出的問題是，為什么會有：

Log actively being archived by another process

如果一個歸檔進程已經開始歸檔日志，為什么另外一個進程又來歸檔日志？

研究了一下，有了一點新的發現。

首先，歸檔進程的數量是受初始化參數log_archive_max_processes控制，初始的，這個參數設置為2，Oracle啟動兩個歸檔進程：

[oracle@jumper bdump]$ ps -ef|grep ora_arc

oracle 11938 1 0 18:24 ? 00:00:00 ora_arc0_eygle

oracle 11940 1 0 18:24 ? 00:00:00 ora_arc1_eygle

oracle 16260 11894 0 20:21 pts/5 00:00:00 grep ora_arc

其中主歸檔進程會持續訪問控制文件：

*** 2007-06-06 20:23:50.510

WAIT #0: nam='rdbms ipc message' ela= 59999293 p1=6000 p2=0 p3=0

WAIT #0: nam='control file sequential read' ela= 76 p1=0 p2=1 p3=1

WAIT #0: nam='control file sequential read' ela= 36 p1=0 p2=370 p3=1

WAIT #0: nam='control file sequential read' ela= 36 p1=0 p2=8 p3=1

WAIT #0: nam='control file sequential read' ela= 30 p1=0 p2=9 p3=1

*** 2007-06-06 20:24:51.950

WAIT #0: nam='rdbms ipc message' ela= 59999344 p1=6000 p2=0 p3=0

WAIT #0: nam='control file sequential read' ela= 72 p1=0 p2=1 p3=1

WAIT #0: nam='control file sequential read' ela= 35 p1=0 p2=370 p3=1

WAIT #0: nam='control file sequential read' ela= 35 p1=0 p2=8 p3=1

WAIT #0: nam='control file sequential read' ela= 29 p1=0 p2=9 p3=1

另外一個歸檔進程以'rdbms ipc message' 事件處于等待：

*** 2007-06-06 20:01:15.830

WAIT #0: nam='rdbms ipc message' ela= 148300048 p1=14831 p2=0 p3=0

a*** 2007-06-06 20:06:23.030

WAIT #0: nam='rdbms ipc message' ela= 299999745 p1=30000 p2=0 p3=0

a*** 2007-06-06 20:11:30.230

WAIT #0: nam='rdbms ipc message' ela= 299999735 p1=30000 p2=0 p3=0

a*** 2007-06-06 20:16:37.430

WAIT #0: nam='rdbms ipc message' ela= 299999779 p1=30000 p2=0 p3=0

a*** 2007-06-06 20:21:44.630

當我們執行alter system archive log current命令時，Oracle Post歸檔進程時，主歸檔進程首先執行歸檔，而次歸檔進程則需要去讀取控制文件，以獲得當前及下一歸檔日志信息：

*** 2007-06-06 19:58:43.970

WAIT #0: nam='rdbms ipc message' ela= 148134746 p1=30000 p2=0 p3=0

WAIT #0: nam='control file sequential read' ela= 36 p1=0 p2=1 p3=1

WAIT #0: nam='control file sequential read' ela= 29 p1=0 p2=369 p3=1

WAIT #0: nam='control file sequential read' ela= 32 p1=0 p2=8 p3=1

WAIT #0: nam='control file sequential read' ela= 29 p1=0 p2=10 p3=1

這就導致次歸檔進程始終會落后于主歸檔進程，會在警告日志中看到如下信息:

Wed Jun 6 19:58:43 2007

ARCH: Evaluating archive log 4 thread 1 sequence 305

ARCH: Beginning to archive log 4 thread 1 sequence 305

Creating archive destination LOG_ARCHIVE_DEST_1: '/opt/oracle/product/9.2.0/dbs/arch1_305.dbf'

Wed Jun 6 19:58:43 2007

ARC1: Evaluating archive log 4 thread 1 sequence 305

ARC1: Unable to archive log 4 thread 1 sequence 305

Log actively being archived by another process

這在正常情況下是不會出現的，如果一個歸檔進程能夠滿足系統需要，第二個進程就無需啟動，而手動switch log則會Post所有歸檔進程。

在Oracle10g中，情況同樣如此:

Wed Jun 6 20:32:57 2007

ARCH: Evaluating archive log 4 thread 1 sequence 75268

Wed Jun 6 20:32:57 2007

ARC0: Evaluating archive log 4 thread 1 sequence 75268

ARC0: Unable to archive log 4 thread 1 sequence 75268

Log actively being archived by another process

Wed Jun 6 20:32:57 2007

ARCH: Beginning to archive log 4 thread 1 sequence 75268 (2074.-1351690850:2074.-1351608674) (mmsdb)

-The End-

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