前几天有位朋友在留言板上提了这样一个问题：

Fri May 25 20:46:06 2007　　//自动备份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　　 //发生日志切换

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　　 //同时启动归档

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　　 //一秒后另一进程启动，失败

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秒后成功归档

ARCH: Completed archiving　log 6 thread 1 sequence 534

Fri May 25 20:46:18 2007　　 //3秒后日志切换

Thread 1 advanced to log sequence 536

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

以上日志是备份过程中产生的，备份脚本如下：

$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;

}

!!

这位朋友提出的问题是，为什么会有：

Log actively being archived by another process

如果一个归档进程已经开始归档日志，为什么另外一个进程又来归档日志？

研究了一下，有了一点新的发现。

首先，归档进程的数量是受初始化参数log_archive_max_processes控制，初始的，这个参数设置为2，Oracle启动两个归档进程：

[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

其中主归档进程会持续访问控制文件：

*** 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

另外一个归档进程以'rdbms ipc message' 事件处于等待：

*** 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

当我们执行alter system archive log current命令时，Oracle Post归档进程时，主归档进程首先执行归档，而次归档进程则需要去读取控制文件，以获得当前及下一归档日志信息：

*** 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

这就导致次归档进程始终会落后于主归档进程，会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

这在正常情况下是不会出现的，如果一个归档进程能够满足系统需要，第二个进程就无需启动，而手动switch log则会Post所有归档进程。

在Oracle10g中，情况同样如此:

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-