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 [email protected] prod >select name ,bytes/1024/1024 from v$sgastat where rownum <6; NAME BYTES/1024/1024 -------------------------- --------------- fixed_sga 1.27443695 buffer_cache 60 log_buffer 6.0078125 kkj jobq wor .003913879 dpslut_kfdsg .000244141 建立一个最小的日志组 15:09:33 [email protected] prod >select group#,sequence#,status,bytes/1024/1024 from v$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 [email protected] prod >create table tb1 as select * from user_objects; Table created. 15:13:48 [email protected] prod >select count(*) from tb1; COUNT(*) ---------- 376832 15:19:16 [email protected] prod >create table tb2 as select * from tb1 where rownum <100000; Table created. 15:20:30 [email protected] prod >create table tb3 as select * from tb1 where rownum <100000; Table created. 4、建立测试脚本 [[email protected] ~]$ 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 $count + 1` done [[email protected] ~]$ 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 $count + 1` done [[email protected] ~]$ 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 $count + 1` done 5、通过3个session，运行脚本 6、查看session event 15:22:08 [email protected] prod >select sid,username ,event from v$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 [email protected] 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 [email protected] 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 [email protected] prod >r 1* select sid,username ,event from v$session where username=‘SCOTT‘ SID USERNAME EVENT ---------- ------------------------------ ---------------------------------------------------------------- 31 SCOTT log file switch completion 41 SCOTT enq: TX - row lock contention 44 SCOTT log file switch completion 45 SCOTT enq: TX - row lock contention 47 SCOTT log file switch completion 7、查看redo latch竞争 15:25:11 [email protected] prod >select name,misses,sleeps,spin_gets,wait_time from v$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 [email protected] 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 [email protected] 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 [email protected] 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 [email protected] prod >/ NAME MISSES SLEEPS SPIN_GETS WAIT_TIME ---------------------------------------------------------------- ---------- ---------- ---------- ---------- redo copy 220 258 0 661577 redo allocation 81 85 0 103697 15:28:29 [email protected] 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。
时间： 2024-11-04 12:35:00