明天借助一个例子持续讲 ASH 的用法。
客户报告：一个 RAC 形成的环境，在 11:20 左右开始解决慢。提供材料：AWR Report
ASH
请求事项：起因确认
解决办法
先简略看一 AWR Report 的总体情况。
・ Node1
DB Time: 967.74 (mins)
・ Node2
DB Time: 414.41 (mins)
・ Node3
DB Time: 354.11 (mins)
・ Node4
DB Time: 460.29 (mins)
・ Node5
DB Time: 551.66 (mins)
依据以上的信息，能够看到 Node1 的“DB Time : 967.74 (mins)”，Node2–Node5 的两倍左右。
所以，咱们能够看看每个 INSTANCE 的“Top 5 Timed Foreground Events”。
・ Node1
Top 5 Timed Foreground Events
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Avg
wait % DB
Event Waits Time(s) (ms) time Wait Class
------------------------------ ------------ ----------- ------ ------ ----------
db file sequential read 4,893,299 26,714 5 46.0 User I/O
log file sync 476,854 6,132 13 10.6 Commit
DB CPU 5,009 8.6
Disk file operations I/O 163,640 4,128 25 7.1 User I/O
gc current block 3-way 2,866,969 3,163 1 5.4 Cluster
・ Node2
Top 5 Timed Foreground Events
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Avg
wait % DB
Event Waits Time(s) (ms) time Wait Class
------------------------------ ------------ ----------- ------ ------ ----------
gc cr block busy 256,891 4,668 18 18.8 Cluster
DB CPU 4,613 18.6
db file sequential read 3,089,328 3,822 1 15.4 User I/O
gc current block 3-way 2,533,718 2,717 1 10.9 Cluster
gc cr grant 2-way 2,424,954 1,698 1 6.8 Cluster
・ Node3
Top 5 Timed Foreground Events
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Avg
wait % DB
Event Waits Time(s) (ms) time Wait Class
------------------------------ ------------ ----------- ------ ------ ----------
DB CPU 4,368 20.6
gc cr block busy 241,547 4,166 17 19.6 Cluster
db file sequential read 2,272,733 2,652 1 12.5 User I/O
gc current block 3-way 2,123,690 2,157 1 10.2 Cluster
Disk file operations I/O 227,537 1,604 7 7.5 User I/O
・ Node4
Top 5 Timed Foreground Events
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Avg
wait % DB
Event Waits Time(s) (ms) time Wait Class
------------------------------ ------------ ----------- ------ ------ ----------
db file sequential read 5,651,562 6,052 1 21.9 User I/O
DB CPU 5,315 19.2
gc cr block busy 195,097 3,457 18 12.5 Cluster
gc cr grant 2-way 4,318,113 2,704 1 9.8 Cluster
gc current block 3-way 2,451,795 2,571 1 9.3 Cluster
・ Node5
Top 5 Timed Foreground Events
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Avg
wait % DB
Event Waits Time(s) (ms) time Wait Class
------------------------------ ------------ ----------- ------ ------ ----------
db file sequential read 5,631,340 6,850 1 20.7 User I/O
DB CPU 5,586 16.9
gc cr block busy 230,530 4,129 18 12.5 Cluster
gc current block 3-way 3,321,250 3,498 1 10.6 Cluster
gc cr grant 2-way 4,412,873 2,792 1 8.4 Cluster
看到这里，因为所有 INSTANCE 的待机 EVENT 都和 I / O 关联，基本上能够判断为是业务解决集中的问题。
上面就须要仔细分析 ASH 数据，找到能证实论断的货色。
首先，咱们看一下分钟单位的 Active Session 数。
◆SQL 文
SQL> select to_char(sample_time,'yyyy/mm/dd hh24:mi'),count(*)
from m_dba_hist_active_sess_history
where instance_number=<&instance_number>
group by to_char(sample_time,'yyyy/mm/dd hh24:mi')
order by to_char(sample_time,'yyyy/mm/dd hh24:mi');
通过下面曲线图，咱们能够清晰的看到在 11:18，所有 INSTANCE 的 Active Session 数都有不同水平的减少。
而后，针对增长水平最大的 Node1 进行进一步的剖析。先来看看 Active Session 的 PROGRAM 散布状况。
通过简略的剖析，咱们能够晓得通过 sqlplus 和 JDBC Thin Client 连上来的 SESSION 数量最大，并且增长幅度最大。
SQL> select to_char(sample_time,'yyyy/mm/dd hh24:mi'),count(*)
from m_dba_hist_active_sess_history
where instance_number=1
and PROGRAM like 'sqlplus%'
group by to_char(sample_time,'yyyy/mm/dd hh24:mi')
having count(*)>10
order by to_char(sample_time,'yyyy/mm/dd hh24:mi');
TO_CHAR(SAMPLE_TIME,'YYYY/MM/DDHH24:MI') COUNT(*)
------------------------------------------------ ----------。。。。。。2021/03/22 11:14 12
2021/03/22 11:15 12
2021/03/22 11:16 12
2021/03/22 11:17 12
2021/03/22 11:18 24 ★SESSION 数减少 1 倍
2021/03/22 11:19 22
2021/03/22 11:20 26
2021/03/22 11:21 19
2021/03/22 11:22 16
2021/03/22 11:23 18
2021/03/22 11:24 17
2021/03/22 11:25 18
2021/03/22 11:26 18
2021/03/22 11:27 20
2021/03/22 11:28 22
2021/03/22 11:29 21
2021/03/22 11:30 23。。。。。。SQL> select to_char(sample_time,'yyyy/mm/dd hh24:mi'),count(*)
2 from m_dba_hist_active_sess_history
3 where instance_number=1
4 and PROGRAM like 'JDBC Thin Client'
5 group by to_char(sample_time,'yyyy/mm/dd hh24:mi')
having count(*)>10
order by to_char(sample_time,'yyyy/mm/dd hh24:mi'); 6 7
TO_CHAR(SAMPLE_TIME,'YYYY/MM/DDHH24:MI') COUNT(*)
------------------------------------------------ ----------。。。。。。2021/03/22 11:13 99
2021/03/22 11:14 104
2021/03/22 11:15 116
2021/03/22 11:16 118
2021/03/22 11:17 119
2021/03/22 11:18 168 ★増加した
2021/03/22 11:19 100
2021/03/22 11:20 100
2021/03/22 11:21 137
2021/03/22 11:22 121
2021/03/22 11:23 134
2021/03/22 11:24 113
2021/03/22 11:25 108
2021/03/22 11:26 119
2021/03/22 11:27 89
2021/03/22 11:28 107
2021/03/22 11:29 100。。。。。。到当初为止，基本上能够断定为 11:18 左右，通过 sqlplus 和 JDBC Thin Client 连上来的 SESSION 数过于集中，
引起了 I / O 解决过多，导致了这次问题的产生。
那这种问题应该如何解决呢？
有上面两个方向：
1. 扩散业务解决。2. 找到 I / O 多的具体解决，看看有没有 I / O 少的办法。第 2 个方向波及 SQL TUNING 的常识，这里不再细说了。
2021/03/23 @ Dalian