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!
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