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Oracle Study之--Oracle等待事件（2）
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Control file parallel write
当数据库中有多个控制文件的拷贝时，Oracle 需要保证信息同步地写到各个控制文件当中，这是一个并行的物理操作过程，因为称为控制文件并行写，当发生这样的操作时，就会产生control file parallel write等待事件。
控制文件频繁写入的原因很多，比如：
· 日志切换太过频繁，导致控制文件信息相应地需要频繁更新。
· 系统I/O 出现瓶颈，导致所有I/O出现等待。
当系统出现日志切换过于频繁的情形时，可以考虑适当地增大日志文件的大小来降低日志切换频率。
当系统出现大量的control file parallel write 等待事件时，可以通过比如降低控制文件的拷贝数量，将控制文件的拷贝存放在不同的物理磁盘上的方式来缓解I/O 争用。
这个等待事件包含三个参数：
Files： Oracle 要写入的控制文件个数。
Blocks： 写入控制文件的数据块数目。
Requests： 写入控制请求的I/O 次数。
Control file sequential read
当数据库需要读取控制文件上的信息时，会出现这个等待事件，因为控制文件的信息是顺序写的，所以读取的时候也是顺序的，因此称为控制文件顺序读，它经常发生在以下情况：
备份控制文件
RAC 环境下不同实例之间控制文件的信息共享
读取控制文件的文件头信息
读取控制文件其他信息
这个等待事件有三个参数：
File#： 要读取信息的控制文件的文件号。
Block#： 读取控制文件信息的起始数据块号。
Blocks： 需要读取的控制文件数据块数目。
模拟案例：
1、模拟事务处理
16:59:48 SYS@ test1>select username,sid,serial# from v$session where username is not null;
USERNAME SID SERIAL#
------------------------------ ---------- ----------
SCOTT 1 7
SYS 34 4
TOM 41 3
16:59:57 SCOTT@ test1>begin
17:04:05 2 for i in 1..1000000 loop
17:04:05 3 execute immediate 'insert into t1(id) values ('||i||')';
17:04:05 4 end loop;
17:04:05 5 end;
17:04:05 6 /
17:00:00 TOM@ test1> begin
17:04:15 2 for i in 1..1000000 loop
17:04:15 3 execute immediate 'insert into scott.t1(id) values ('||i||')';
17:04:15 4 end loop;
17:04:15 5 end;
17:04:15 6 /2、查看等待事件：
17:05:40 SYS@ test1>select sid,event,p1,p2,p3,WAIT_TIME from v$session_wait where sid in (1,41); SID EVENT P1 P2 P3 WAIT_TIME ---------- -------------------------------------------------- ---------- ---------- ----- 1 latch: redo allocation 1993106152 182 0 -1 41 latch: cache buffers chains 2011026752 150 0 -1 Elapsed: 00:00:00.01 17:06:11 SYS@ test1>r 1* select sid,event,p1,p2,p3,WAIT_TIME from v$session_wait where sid in (1,41) SID EVENT P1 P2 P3 WAIT_TIME ---------- -------------------------------------------------- ---------- ---------- ------- 1 log file switch (checkpoint incomplete) 0 0 0 3 41 buffer busy waits 4 4048 1 94 Elapsed: 00:00:00.01 17:06:16 SYS@ test1>r 1* select sid,event,p1,p2,p3,WAIT_TIME from v$session_wait where sid in (1,41) SID EVENT P1 P2 P3 WAIT_TIME ---------- -------------------------------------------------- ---------- ---------- ----- 1 log file switch (checkpoint incomplete) 0 0 0 0 41 log file switch (checkpoint incomplete) 0 0 0 0 Elapsed: 00:00:00.02 17:06:18 SYS@ test1>r 1* select sid,event,p1,p2,p3,WAIT_TIME from v$session_wait where sid in (1,41) SID EVENT P1 P2 P3 WAIT_TIME ---------- -------------------------------------------------- ---------- ---------- ---------- ---------- 1 log buffer space 0 0 0 0 41 log buffer space 0 0 0 0 Elapsed: 00:00:00.01 17:06:20 SYS@ test1>r 1* select sid,event,p1,p2,p3,WAIT_TIME from v$session_wait where sid in (1,41) SID EVENT P1 P2 P3 WAIT_TIME ---------- -------------------------------------------------- ---------- ---------- ----- 1 log file switch (checkpoint incomplete) 0 0 0 134 41 log file switch (checkpoint incomplete) 0 0 0 3 Elapsed: 00:00:00.01 17:06:22 SYS@ test1>r 1* select sid,event,p1,p2,p3,WAIT_TIME from v$session_wait where sid in (1,41) SID EVENT P1 P2 P3 WAIT_TIME ---------- -------------------------------------------------- ---------- ---------- ---------- ---------- 1 latch: cache buffers chains 2011026752 150 0 -1 41 latch: cache buffers chains 2011026752 150 0 -1 Elapsed: 00:00:00.01
3、查看关于controlfile的事件
17:06:24 SYS@ test1>select EVENT ,TOTAL_WAITS,TIME_WAITED from v$system_event where event like '%control%'; EVENT TOTAL_WAITS TIME_WAITED -------------------------------------------------- ----------- ----------- control file sequential read 51623 326 control file parallel write 6327 5102 control file heartbeat 1 400 Elapsed: 00:00:00.02 17:07:31 SYS@ test1>r 1* select EVENT ,TOTAL_WAITS,TIME_WAITED from v$system_event where event like '%control%' EVENT TOTAL_WAITS TIME_WAITED -------------------------------------------------- ----------- ----------- control file sequential read 52314 329 control file parallel write 6410 5164 control file heartbeat 1 400 Elapsed: 00:00:00.03 17:07:41 SYS@ test1>r 1* select EVENT ,TOTAL_WAITS,TIME_WAITED from v$system_event where event like '%control%' EVENT TOTAL_WAITS TIME_WAITED -------------------------------------------------- ----------- ----------- control file sequential read 52597 330 control file parallel write 6433 5177 control file heartbeat 1 400 Elapsed: 00:00:00.02
4、查看告警日志：
Beginning log switch checkpoint up to RBA [0x438.2.10], SCN: 1762466 Thread 1 advanced to log sequence 1080 (LGWR switch) Current log# 3 seq# 1080 mem# 0: /u01/app/oracle/oradata/redo03a.log Fri Jul 25 17:10:26 2014 Archived Log entry 1065 added for thread 1 sequence 1079 ID 0x4906548b dest 1: Thread 1 cannot allocate new log, sequence 1081 Checkpoint not complete Current log# 3 seq# 1080 mem# 0: /u01/app/oracle/oradata/redo03a.log Completed checkpoint up to RBA [0x438.2.10], SCN: 1762466 Beginning log switch checkpoint up to RBA [0x439.2.10], SCN: 1762522 Thread 1 advanced to log sequence 1081 (LGWR switch) Current log# 4 seq# 1081 mem# 0: /u01/app/oracle/oradata/redo04a.log Fri Jul 25 17:10:29 2014 Archived Log entry 1066 added for thread 1 sequence 1080 ID 0x4906548b dest 1: Thread 1 cannot allocate new log, sequence 1082 Checkpoint not complete Current log# 4 seq# 1081 mem# 0: /u01/app/oracle/oradata/redo04a.log Completed checkpoint up to RBA [0x439.2.10], SCN: 1762522 Fri Jul 25 17:10:32 2014 Beginning log switch checkpoint up to RBA [0x43a.2.10], SCN: 1762575 Thread 1 advanced to log sequence 1082 (LGWR switch) Current log# 3 seq# 1082 mem# 0: /u01/app/oracle/oradata/redo03a.log Archived Log entry 1067 added for thread 1 sequence 1081 ID 0x4906548b dest 1:
5、查看日志组信息
17:16:22 SYS@ test1>select group#,sequence#,status,bytes/1024/1024 from v$log; GROUP# SEQUENCE# STATUS BYTES/1024/1024 ---------- ---------- ---------------- --------------- 3 1082 CURRENT 4 4 1081 INACTIVE 4 Elapsed: 00:00:00.03 日志组size太小，日志组数量较少，日志切换太频繁，导致产生大量的控制文件读和写，调整日志组的size，并增加日志组的个数。