本文转自 https://blogs.oracle.com/database4cn/log-file-switch-checkpoint-incomplete-%e5%ae%b9%e6%98%93%e8%a2%ab%e8%af%af%e8%af%8a%e7%9a%84event
 
首先来看如下的一份AWR，12分钟的采样，DB Time 105分钟。
DB Name DB Id Instance Inst num Startup Time Release RAC

R11204 2114874159 R11204 1 23-Oct-17 10:10 11.2.0.4.0 NO

Host Name Platform CPUs Cores Sockets Memory (GB)

nascds18 Linux x86 64-bit 2 2 1 11.64

Snap Id Snap Time Sessions Cursors/Session

Begin Snap: 3 23-Oct-17 10:55:46 37 2.5

End Snap: 4 23-Oct-17 11:08:27 53 2.3

Elapsed:   12.67 (mins)

DB Time:   105.90 (mins)
 
Top event 发现 buffer busy waits 和 log file switch (checkpoint incomplete) 几乎占用了所有的DB Time
Top 10 Foreground Events by Total Wait Time

Event Waits Total Wait Time (sec) Wait Avg(ms) % DB time Wait Class

buffer busy waits 11 3310.6 300960 52.1 Concurrency

log file switch (checkpoint incomplete) 10 3034.8 303479 47.8 Configuration

DB CPU   5.5   .1

log file sync 28 2.3 82 .0 Commit

log buffer space 24 .8 32 .0 Configuration
通过ASH不难发现 buffer busy waits 被 log file switch (checkpoint incomplete) 阻塞，而 log file switch (checkpoint incomplete) 又被 LGWR 的 control file sequential read 阻塞。
2017-10-23 11:05:31.563 1 37 perl@nascds18 (TNS V1-V3) buffer busy waits WAITING 1 150

2017-10-23 11:05:31.563 1 150 sqlplus@nascds18 (TNS V1-V3) buffer busy waits WAITING 1 141

2017-10-23 11:05:31.563 1 141 OMS log file switch (checkpoint incomplete) WAITING 1 130

2017-10-23 11:05:31.563 1 130 oracle@nascds18 (LGWR) control file sequential read WAITING NO HOLDER
接下来再看一眼AWR的iostat，很快发现LGWR有大量的读取，并且都是在读取控制控制文件，12分钟内的读取高达1.5G。
IOStat by Function summary

Function Name Reads: Data

LGWR 1.5G

Others 210M

DBWR 0M

Buffer Cache Reads 10M

Direct Writes 0M

TOTAL: 1.7G

IOStat by Filetype summary

Filetype Name Reads: Data

Control File 1.5G

Log File 185M

Archive Log 0M

Data File 10M

Temp File 0M

TOTAL: 1.7G
结合ASH中的 LGWR 是最终holder，并且 LGWR 等待 control file sequential read 的事实，我们可能很快将 LGWR 列为重点怀疑对象，如果真是这样，那就跑偏了。
 
此时不妨停下来思考一下，什么是 log file switch (checkpoint incomplete)？
 
log file switch (checkpoint incomplete) 指的是当redo需要向下一组 redo group 切换的时候，发现下组日志是 active 的，也就是说下组日志中对应的一些 buffer cache 中的脏块尚未写入到数据文件中，因此必须等待这些脏块被完毕后，才可以复用下一组 redo group。
 
接下来再思考一下，哪个进程负责将脏块写入到数据文件？答案是DBWn，因此我们更需要关注的是DBWn。分析一下OSWatcher不难发现DBW0的状态是D，man一下ps命令: D指的是Uninterruptible sleep (usually IO)，也就是说DBW0 hang在I/O上了，这才是问题的根本原因。
 
那么为什么LGWR会执行如此多的 control file sequential read ？答案是DBWn出现问题导致 LGWR不断查询control 文件获取redo 状态(看它有没有切换成功)导致大量的 control file sequential read
 
下面通过实验来演示这个现象:
 
Test on 12.1.0.2
 
Session 1:  创建一个表，并且update一下让其产生redo
Connected to:

Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit Production

With the Partitioning, OLAP, Advanced Analytics and Real Application Testing options

SQL> select sid from v$mystat where rownum=1;

      SID

----------

       11   <<<< session 1的 sid 为11

SQL> create table t as select * from dba_objects;

Table created.

SQL> update t set object_name=object_name;

93841 rows updated.
Session 2:  用于查看redo的切换状态
SQL> set line 200 pages 1000

SQL> select group#,thread#,status from v$log;

   GROUP#    THREAD# STATUS

---------- ---------- ----------------

        1          1 INACTIVE

        2          1 INACTIVE

        3          1 CURRENT

SQL> alter system switch logfile;

System altered.

SQL> alter system switch logfile;

System altered.

SQL> select group#,thread#,status from v$log;

   GROUP#    THREAD# STATUS

---------- ---------- ----------------

        1          1 ACTIVE     <<<<<< 让下一组是active的

        2          1 ACTIVE

        3          1 CURRENT
Session 3: 确定DBWn以及LGWR的 spid，并且通过 oradebug 将 DBW0 suspend住让其不工作
SQL> set line 200 pages 1000

SQL> select spid,program from v$process where program like '%DBW%' or program like '%LG%';

SPID                     PROGRAM

------------------------ ----------------------------------------------------------------

5768                     ORACLE.EXE (DBW0)

7248                     ORACLE.EXE (LGWR)

6384                     ORACLE.EXE (LG00)

6308                     ORACLE.EXE (LG01)

SQL> oradebug setospid 5768

Oracle pid: 11, Windows thread id: 5768, image: ORACLE.EXE (DBW0)

SQL> oradebug suspend

Statement processed.
接下来在Session 1中再执行几次update让其自动切换日志:
Session 1:
SQL> update t set object_name=object_name;

93841 rows updated.

SQL> /

93841 rows updated.

SQL> /

93841 rows updated.

SQL> /   ---- 在这里hang住了
Session 4: 观察等待事件
SQL> set line 200 pages 1000

SQL> col program for a15

SQL> col event for a40

SQL> select sid,serial#,program,event,state from v$session where program like '%sqlplus%';

      SID    SERIAL# PROGRAM         EVENT                                    STATE

---------- ---------- --------------- ---------------------------------------- -------------------

       10      33682 sqlplus.exe     SQL*Net message from client              WAITING

       11      48189 sqlplus.exe     log file switch (checkpoint incomplete)  WAITING

      129      25471 sqlplus.exe     SQL*Net message to client                WAITED SHORT TIME

      130      64963 sqlplus.exe     SQL*Net message from client              WAITING
接下来我们对LGWR做10046 trace，观察其等待: 可见反复的 control file sequential read
SQL> oradebug setospid 7248

Oracle pid: 12, Windows thread id: 7248, image: ORACLE.EXE (LGWR)

SQL> oradebug event 10046 trace name context forever, level 8

Statement processed.

SQL> oradebug tracefile_name

D:\ORACLE\diag\rdbms\r12102\r12102\trace\r12102_lgwr_7248.trc

tail -f D:\ORACLE\diag\rdbms\r12102\r12102\trace\r12102_lgwr_7248.trc

WAIT #0: nam='LGWR all worker groups' ela= 72 p1=0 p2=0 p3=0 obj#=-1 tim=25178622234

WAIT #0: nam='control file sequential read' ela= 407 file#=0 block#=1 blocks=1 obj#=-1 tim=25178622880

WAIT #0: nam='control file sequential read' ela= 262 file#=1 block#=1 blocks=1 obj#=-1 tim=25178623344

WAIT #0: nam='control file sequential read' ela= 717 file#=0 block#=15 blocks=1 obj#=-1 tim=25178624315

WAIT #0: nam='control file sequential read' ela= 1774 file#=0 block#=17 blocks=1 obj#=-1 tim=25178626427

WAIT #0: nam='control file sequential read' ela= 311 file#=0 block#=19 blocks=1 obj#=-1 tim=25178627527

WAIT #0: nam='control file sequential read' ela= 269 file#=0 block#=284 blocks=1 obj#=-1 tim=25178627983

WAIT #0: nam='control file sequential read' ela= 238 file#=0 block#=22 blocks=1 obj#=-1 tim=25178628363

WAIT #0: nam='LGWR all worker groups' ela= 51 p1=0 p2=0 p3=0 obj#=-1 tim=25178628590

WAIT #0: nam='control file sequential read' ela= 503 file#=0 block#=1 blocks=1 obj#=-1 tim=25178629320

WAIT #0: nam='control file sequential read' ela= 322 file#=1 block#=1 blocks=1 obj#=-1 tim=25178630389

WAIT #0: nam='control file sequential read' ela= 276 file#=0 block#=15 blocks=1 obj#=-1 tim=25178630864

WAIT #0: nam='control file sequential read' ela= 253 file#=0 block#=17 blocks=1 obj#=-1 tim=25178631286

WAIT #0: nam='control file sequential read' ela= 250 file#=0 block#=19 blocks=1 obj#=-1 tim=25178631696

WAIT #0: nam='control file sequential read' ela= 658 file#=0 block#=284 blocks=1 obj#=-1 tim=25178632935

WAIT #0: nam='control file sequential read' ela= 303 file#=0 block#=22 blocks=1 obj#=-1 tim=25178633812

......
最后我们将DBWn恢复，结束这个无限的 control file sequential read 等待。
回到Session 3，resume DBW0：
SQL> oradebug resume

Statement processed.
只要理解了原理，这个问题本来不难诊断，但是ASH和AWR的信息会让DBA错误的认为是 LGWR 的等待control file sequential read导致的问题。

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