ORACLE 数据库11.2.0.4 单实例服务器IO等待高问题分析。分析过程中发现一条SQL消耗IO高的SQL，进一步分析发现有全表扫描，另外发现主机IO-WAIT高的时间段内有备份作业在执行，初步确定是备份作业和SQL全表扫描导致主机IO-wait高，触发告警。
1、问题描述
20180929，客户反馈服务器磁盘IOWAIT 较高，一般在10以上，高的时候，可达30%以上。
2、提取分析日志，数据库告警日志、问题时段的AWR、及问题SQL的sqlhc、系统IOTOP监控记录
3、问题分析
a、观察数据库告警日志，没有发现异常的数据库ORA报错
b、观察系统IOTOP监控记录发现问题时段有数据库备份任务在执行：
c、观察问题时段数据库的AWR发现一条又性能问题的SQL语句：
SQL语句文本如下：
| select a.inter_code, a.hq_date, a.repair_unit_nav as unit_nav from dm_fund_hq a, dm_stock_info b where a.inter_code = b.inter_code and b.fund_style in (100301, 100101, 100201) and a.hq_date > to_char(add_months(sysdate, -12), 'yyyyMMdd') and a.inter_code = :1 order by a.hq_date |
SQL执行计划如下：
根据oracle的SQL自动调优，获得如下建议：
FINDINGS SECTION (2 findings)
-------------------------------------------------------------------------------
1- SQL Profile Finding (see explain plans section below)
--------------------------------------------------------
A potentially better execution plan was found for this statement.
Recommendation (estimated benefit: 86.11%)
------------------------------------------
- Consider accepting the recommended SQL profile to use parallel execution
for this statement.
execute dbms_sqltune.accept_sql_profile(task_name => 'TASK_16308',
task_owner => 'SYS', replace => TRUE, profile_type =>
DBMS_SQLTUNE.PX_PROFILE);
Executing this query parallel with DOP 8 will improve its response time
86.11% over the original plan. However, there is some cost in enabling
parallel execution. It will increase the statement's resource consumption by
an estimated 11.11% which may result in a reduction of system throughput.
Also, because these resources are consumed over a much smaller duration, the
response time of concurrent statements might be negatively impacted if
sufficient hardware capacity is not available.
The following data shows some sampled statistics for this SQL from the past
week and projected weekly values when parallel execution is enabled.
Past week sampled statistics for this SQL
-----------------------------------------
Number of executions 0
Percent of total activity 0
Percent of samples with #Active Sessions > 2*CPU 0
Weekly DB time (in sec) 0
Projected statistics with Parallel Execution
--------------------------------------------
Weekly DB time (in sec) 0
2- Index Finding (see explain plans section below)
--------------------------------------------------
The execution plan of this statement can be improved by creating one or more
indices.
Recommendation (estimated benefit: 99.98%)
------------------------------------------
- Consider running the Access Advisor to improve the physical schema design
or creating the recommended index.
create index ETSPRD.IDX$$_3FB40001 on ETSPRD.DM_STOCK_INFO("INTER_CODE");
- Consider running the Access Advisor to improve the physical schema design
or creating the recommended index.
create index ETSPRD.IDX$$_3FB40002 on
ETSPRD.DM_FUND_HQ("INTER_CODE","HQ_DATE","REPAIR_UNIT_NAV");
Rationale
---------
Creating the recommended indices significantly improves the execution plan
of this statement. However, it might be preferable to run "Access Advisor"
using a representative SQL workload as opposed to a single statement. This
will allow to get comprehensive index recommendations which takes into
account index maintenance overhead and additional space consumption.
4、分析总结
a、问题时段有备份任务执行，消耗系统服务器CPU、IO资源；
b、观察问题时段AWR发现一条可优化的SQL语句，相关优化措施见问题分析。