通过案例学调优之--和 LOG BUFFER 相关的主要 Latch
4.1、和 LOG BUFFER 相关的主要 Latch
有: Latch:Redo Copy
Latch:Redo Allocation Latch
4.2 当一个进程在修改数据时候将会产生 Redo,这个 Redo 首先在 PGA 中保存。
然后进程需要 获取Redo Copy Latch(这个Latch的个数由隐含参数_log_simultaneous_copies决定),当获 得 Redo Copy Latch 后,进程接着获取 Redo Allocation Latch 来分配 Redo Log Buffer 中的空间, 空间分配完成后,释放 Redo Allocation Latch。然后进程把 PGA 里临时存放的 Redo 信息复制 到 Redo Log Buffer,复制完成后,释放 Redo Copy Latch。
4.3 逻辑架构如下:
案例分析：
测试redo中Latch的竞争
1、建立测试环境
15:08:51 SYS@ prod >select name ,bytes/1024/1024 from v$sgastat where rownum <6;
NAME BYTES/1024/1024
-------------------------- ---------------
fixed_sga 1.27443695
buffer_cache 60
log_buffer 6.0078125
kkj jobq wor .003913879
dpslut_kfdsg .000244141
建立一个最小的日志组
15:09:33 SYS@ prod >select group#,sequence#,status,bytes/1024/1024 from v$log;
GROUP# SEQUENCE# STATUS BYTES/1024/1024
---------- ---------- ---------------- ---------------
4 108 CURRENT 4
5 106 INACTIVE 4
2、建立三张测试表
15:11:59 SCOTT@ prod >create table tb1 as select * from user_objects;
Table created.
15:13:48 SCOTT@ prod >select count(*) from tb1;
COUNT(*)
----------
376832
15:19:16 SCOTT@ prod >create table tb2 as select * from tb1 where rownum <100000;
Table created.
15:20:30 SCOTT@ prod >create table tb3 as select * from tb1 where rownum <100000;
Table created.
4、建立测试脚本
[oracle@RH6 ~]$ cat 22.sh
#!/bin/bash
export ORACLE_SID=prod
count=0
while [ $count -lt 1000 ]
do
sqlplus 'scott/tiger'<<EOF
update tb1 set object_id=1000 ;
rollback;
EOF
count=`expr $count + 1`
done
[oracle@RH6 ~]$ cat 33.sh
#!/bin/bash
export ORACLE_SID=prod
count=0
while [ $count -lt 1000 ]
do
sqlplus 'scott/tiger'<<EOF
update tb2 set object_id=1000 ;
rollback;
EOF
count=`expr $count + 1`
done
[oracle@RH6 ~]$ cat 44.sh
#!/bin/bash
export ORACLE_SID=prod
count=0
while [ $count -lt 1000 ]
do
sqlplus 'scott/tiger'<<EOF
update tb3 set object_id=1000 ;
rollback;
EOF
count=`expr $count + 1`
done
5、通过3个session，运行脚本
6、查看session event
15:22:08 SYS@ prod >select sid,username ,event from v$session where username='SCOTT';
SID USERNAME EVENT
---------- ------------------------------ ----------------------------------------------------------------
31 SCOTT log file switch (checkpoint incomplete)
45 SCOTT enq: TX - row lock contention
Elapsed: 00:00:00.00
15:22:14 SYS@ prod >/
SID USERNAME EVENT
---------- ------------------------------ ----------------------------------------------------------------
31 SCOTT log file switch completion
41 SCOTT enq: TX - row lock contention
44 SCOTT log file switch completion
45 SCOTT enq: TX - row lock contention
47 SCOTT log file switch completion
15:23:42 SYS@ prod >/
SID USERNAME EVENT
---------- ------------------------------ ----------------------------------------------------------------
31 SCOTT db file sequential read
41 SCOTT enq: TX - row lock contention
44 SCOTT latch: redo copy
45 SCOTT enq: TX - row lock contention
47 SCOTT latch: redo allocation
15:26:54 SYS@ prod >r
1* select sid,username ,event from v$session where username='SCOTT'
SID USERNAME EVENT
---------- ------------------------------ ----------------------------------------------------------------
31 SCOTT log file switch completion
41 SCOTT enq: TX - row lock contention
44 SCOTT log file switch completion
45 SCOTT enq: TX - row lock contention
47 SCOTT log file switch completion
7、查看redo latch竞争
15:25:11 SYS@ prod >select name,misses,sleeps,spin_gets,wait_time from v$latch
15:25:34 2 where name in ('redo copy','redo allocation');
NAME MISSES SLEEPS SPIN_GETS WAIT_TIME
---------------------------------------------------------------- ---------- ---------- ---------- ----------
redo copy 101 116 0 279828
redo allocation 48 50 0 54560
Elapsed: 00:00:00.02
15:25:53 SYS@ prod >/
NAME MISSES SLEEPS SPIN_GETS WAIT_TIME
---------------------------------------------------------------- ---------- ---------- ---------- ----------
redo copy 111 126 0 300388
redo allocation 50 52 0 56124
Elapsed: 00:00:00.01
15:26:08 SYS@ prod >/
NAME MISSES SLEEPS SPIN_GETS WAIT_TIME
---------------------------------------------------------------- ---------- ---------- ---------- ----------
redo copy 111 126 0 300388
redo allocation 50 52 0 56124
Elapsed: 00:00:00.00
15:26:12 SYS@ prod >/
NAME MISSES SLEEPS SPIN_GETS WAIT_TIME
---------------------------------------------------------------- ---------- ---------- ---------- ----------
redo copy 202 234 0 594703
redo allocation 75 79 0 83114
Elapsed: 00:00:00.00
15:27:58 SYS@ prod >/
NAME MISSES SLEEPS SPIN_GETS WAIT_TIME
---------------------------------------------------------------- ---------- ---------- ---------- ----------
redo copy 220 258 0 661577
redo allocation 81 85 0 103697
15:28:29 SYS@ prod >/
NAME MISSES SLEEPS SPIN_GETS WAIT_TIME
---------------------------------------------------------------- ---------- ---------- ---------- ----------
redo copy 346 400 1 1174583
redo allocation 146 150 0 189359可以看到，在系统中产生了大量的redo latch的争用。
