逻辑读产生Cache Buffer Chain(简称CBC) Latch的解析

测试环境：版本11gR2

SQL> select * from v$version where rownum=1;

BANNER

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

Oracle Database 11g Enterprise Edition Release 11.2.0.1.0 - 64bit Production

一、逻辑读的过程

1、Oracle以每个块的文件号、块号和类型做HASH运算,得到HASH值。根据HASH值，到HASH表中取出指定块的内存地址

2、获取CBC Latch(实验的重点测试部分)

3、根据HASH值,搜索CBC链表

4、根据DBA找到BH(Buffer Header)加Buffer Pin

5、加完Buffer Pin马上释放CBC Latch

6、访问Buffer开始fetch数据

7、获取CBC Latch

8、释放Buffer Pin

9、释放CBC Latch

二、取T1表的第一行数据及ROWID，根据dbms_rowid包查出这行数据的文件号、块号

SQL> select rowid,dbms_rowid.rowid_relative_fno(rowid) file#,dbms_rowid.rowid_block_number(rowid) block#,id,name from gyj.t1　where rownum=1;

ROWID FILE# BLOCK# ID NAME

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

AAASP9AAGAAAACHAAA 6 135 1 aaaaa

这里的DBA(Data Block Address)就是由6号文件和135号块组成

三、根据文件号块号获取CBC Latch的地址

SQL> select hladdr from x$bh where file#=6 and dbablk=135;

　　HLADDR

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

00000003A4282A90

四、根据CBC Latch的地址可以查出这个CBC Latch被获得的次数

1.获取CBC Latch的次数为245

SQL> select addr,name,gets from v$latch_children where addr='00000003A4282A90';

ADDR NAME GETS

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

00000003B09F7C58 cache buffers chains 245

2.再次读取T1表的第一数据，即产生一次逻辑读

SQL>select id,name from gyj.t1 where rowid='AAASP9AAGAAAACHAAA';

ID　　　　　　NAME

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

　　　　1　　　　　gyj1

3.CBC Latch的次数变为247，说明一次逻辑读产生两次CBC Latch

SQL> select addr,name,gets from v$latch_children where addr='00000003A4282A90';

ADDR NAME GETS

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

00000003A4282A90 cache buffers chains 247

五、查本会话下的进程号

SQL> select spid from v$session s,v$process b where s.paddr=b.addr and s.sid in(select sid from v$mystat where rownum=1);

SPID

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

2416

六、利用Dtrace工具跟踪

１.编写个简单的dtrace脚本

vi dtrace.d

#!/usr/sbin/dtrace -s -n

dtrace:::BEGIN

{

i=1;

}

pid$1:::entry

{

printf("i=%d PID::entry:==%s:%s:%s:%s %x %x %x %d %x %x",i, probeprov, probemod, probefunc, probename,arg0,arg1,arg2,arg3,arg4,arg5);

i=i+1;

}

２.授权限

chmod 755 dtrace.d

３.执行dtrace命令生成跟踪日志

./dtrace.d -x switchrate=1hz -b 32m 2416　> dtracecbc.log --用这条命令加大了缓存,避免丢失日志

４.显示主要dtrace部分日志，在dtracecbc.log中搜索latch地址:3A4282A90

CPU ID FUNCTION:NAME

--逻辑读开始

1 63919 kcbgtcr:entry i=592 PID::entry:==pid2416:oracle:kcbgtcr:entry fffffd7ffc98bc00 0 2fe 0 23dede0 fffffd7fffdfa7a0

--获取CBC Latch

1 128720 sskgslcas:entry i=593 PID::entry:==pid2416:oracle:sskgslcas:entry 3a4282a90 0 1 0 0 3a4c953d0　　　　　　　　　　

1 60714 ktrexf:entry i=594 PID::entry:==pid2416:oracle:ktrexf:entry fffffd7fffdfa7a0 d7fcd60 0 15392235540 1fe8 23dc9f0

1 64185 kcbzgs:entry i=595 PID::entry:==pid2416:oracle:kcbzgs:entry 1 d7fcd60 0 0 0 23dc9f0

1 53939 kssadf_numa_intl:entry i=596 PID::entry:==pid2416:oracle:kssadf_numa_intl:entry 23 3a4145900 3a487d9e8 0 0 1d

--根据DBA找到BH(Buffer Header)加Buffer Pin

1 128720 sskgslcas:entry i=597 PID::entry:==pid2416:oracle:sskgslcas:entry 3957f6280 0 1 0 3957f6238 3957f6238

--加完Buffer Pin马上释放CBC Latch

1 128723 sskgsldecr:entry i=598 PID::entry:==pid2416:oracle:sskgsldecr:entry 3a4282a90 1 1 0 1 2000000000000000

1 63951 kcbcge:entry i=599 PID::entry:==pid2416:oracle:kcbcge:entry fffffd7ffc98bc00 0 77 0 fffffd7ffc98b7dc 0

1 64116 kcbds2pbh:entry i=600 PID::entry:==pid2416:oracle:kcbds2pbh:entry fffffd7ffc98bc00 0 77 0 fffffd7ffc98b7dc 0

1 60179 ktcckv:entry i=601 PID::entry:==pid2416:oracle:ktcckv:entry fffffd7ffc98bc00 fffffd7ffc98b7d4 77 0 3a3c935f0 0

1 104774 kafger:entry i=602 PID::entry:==pid2416:oracle:kafger:entry 39572e064 fffffd7fffdfb200 39d84e4f0 2 1 1

1 83919 kpofcr:entry i=603 PID::entry:==pid2416:oracle:kpofcr:entry fffffd7fffdfb4b0 1 39d84e4f0 2 4c30d40 3a4fd3790

1 124023 ttcrxh:entry i=604 PID::entry:==pid2416:oracle:ttcrxh:entry d8055e8 d7f58f0 fffffd7fffdfafe0 48 214 1

1 209006 memcpy:entry i=605 PID::entry:==pid2416:libc.so.1:memcpy:entry ddfa329 fffffd7fffdfafe0 30 48 ddfa359 d7f5a00

1 209006 memcpy:entry i=606 PID::entry:==pid2416:libc.so.1:memcpy:entry ddfa35b 39572fff5 2 2 2 d7f5a00

1 123964 ttcc2u:entry i=607 PID::entry:==pid2416:oracle:ttcc2u:entry d8055e8 d7f58f0 39572fff8 4 1 1

1 209006 memcpy:entry i=608 PID::entry:==pid2416:libc.so.1:memcpy:entry ddfa35e 39572fff8 4 852 d7f5a00 d7f5a00

1 61112 ktsmg_max_query:entry i=609 PID::entry:==pid2416:oracle:ktsmg_max_query:entry 0 0 39572fffc 0 d805250 2

1 211309 gethrtime:entry i=610 PID::entry:==pid2416:libc.so.1:gethrtime:entry 92d 1a 39572fffc 8 3a4447d80 39de97dc8

1 128314 slcpums:entry i=611 PID::entry:==pid2416:oracle:slcpums:entry d805218 1a 494b3f016 8 3a443a460 39c253698

1 211401 times:entry i=612 PID::entry:==pid2416:libc.so.1:times:entry fffffd7fffdfb240 1a 494b3f016 8 3a443a460 39c253698

1 131260 kglHandleInvalidations:entry i=613 PID::entry:==pid2416:oracle:kglHandleInvalidations:entry d8055e8 39df04bb0 0 8 d7f1a10 39c2d1140

1 131261 kglHandleLoads:entry i=614 PID::entry:==pid2416:oracle:kglHandleLoads:entry d8055e8 39df04bb0 0 8 d7f1a10 39c2d1140

1 91756 kksGetStats:entry i=615 PID::entry:==pid2416:oracle:kksGetStats:entry 39d738198 39df04bb0 0 8 d7f1a10 39c2d1140

1 72794 kews_sqlcol_end:entry i=616 PID::entry:==pid2416:oracle:kews_sqlcol_end:entry 3a4fd2210 fffffd7ffc9e1ad8 39d737e88 70 1 1

1 110094 qecrlssub:entry i=617 PID::entry:==pid2416:oracle:qecrlssub:entry 39d84ed10 fffffd7ffc9e1ad8 9 0 fffffd7ffc98ac58 0

1 108487 qertbRelease:entry i=618 PID::entry:==pid2416:oracle:qertbRelease:entry 39d84dea8 fffffd7ffc98bae0 9 0 4c0 dbb56b0

1 104779 kafcpy:entry i=619 PID::entry:==pid2416:oracle:kafcpy:entry 39d84e4f0 2 9 0 fffffd7ffc987430 20

--访问Buffer开始fetch数据

1 209006 memcpy:entry i=620 PID::entry:==pid2416:libc.so.1:memcpy:entry fffffd7ffc9f49f8 39572fff8 4 0 30 20

1 209006 memcpy:entry i=621 PID::entry:==pid2416:libc.so.1:memcpy:entry fffffd7ffc9f49e0 39572fff5 2 1 30 20

1 63927 kcbipnns:entry i=622 PID::entry:==pid2416:oracle:kcbipnns:entry fffffd7ffc98bc00 39572fff5 39572fff7 0 fffffd7ffc98bbe8 2c1

--释放Buffer Pin，调用函数sskgslcas(目的是获取CBC Latch)

1 63940 kcbrls:entry i=623 PID::entry:==pid2416:oracle:kcbrls:entry fffffd7ffc98bc00 39572fff5 39572fff7 0 3a3c935f0 2c1

--获取CBC Latch

1 128720 sskgslcas:entry i=624 PID::entry:==pid2416:oracle:sskgslcas:entry 3a4282a90 0 200000000000001d 9 3a4c95288 2000000000000000

1 64198 kcbzar:entry i=625 PID::entry:==pid2416:oracle:kcbzar:entry 3a3c93670 3a4282a90 108000 -2748781186960 0 10

--释放CBC Latch

1 128723 sskgsldecr:entry i=626 PID::entry:==pid2416:oracle:sskgsldecr:entry 3a4282a90 200000000000001d fffffffd 1 2000000000000000 1d

５.利用oradebug工具，逻辑读获取CBC Latch

SQL> oradebug setmypid

Statement processed.

SQL> oradebug peek 0x3a4282a90 4 --查0x3a4282a90地址开始的4字节信息的值为0

[3A4282A90, 3A4282A94) = 00000000

SQL> oradebug poke 0x3a4282a90 4 1　　 　 --修改0x3a4282a90地址开始的4字节信息的值为1,相当于获取了Latch

BEFORE: [3A4282A90, 3A4282A94) = 00000000 --修改前的值

AFTER: [3A4282A90, 3A4282A94) = 00000001 --修改后的值

SQL> oradebug peek 0x3a4282a90 4　　　 --查0x3a4282a90地址开始的4字节信息的值为1　　

[3A4282A90, 3A4282A94) = 00000001

６.再开一个会话,会话号为13：

select sid from v$mystat where rownum=1;

SID

----------

13

７.在13会话下再查询T1表的第一行，这时已被阻塞

select id,name from gyj.t1 where rowid='AAASP9AAGAAAACHAAA';

８.再开一个会话，查看会话号是13的等待事件，产生了CBC Latch的等待事件

select sid,event,p1raw,p2raw,p3raw from v$session where sid=13;

SID EVENT P1RAW P2RAW P3RAW

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

13 latch: cache buffers chains 00000003A4282A90 0000000000000096 00

这样跟踪一下，就很清楚为什么一次逻辑读要加两次CBC Latch，一次为了加Buffer Pin，一次为了释放Buffer Pin!