Higher 'direct path read' Waits in 11g when Compared to 10g (Doc ID 793845.1)
2014-11-05 22:26
543 查看
In this Document
Oracle Database - Enterprise Edition - Version 11.1.0.6 and later
Information in this document applies to any platform.
Intermittent but noticeable increase in 'direct path read' waits were observed in 11g when compared to Oracle 10g. For example, top 5 events:
Typically these will be higher at times where many serial Full Table Scans of Tables occur
Using Automatic Shared Memory Management (ASMM)
Database upgraded from 10g to 11g
Database upgraded from 10g to 11g
In 10g, serial table scans for "large" tables go through the buffer cache (by default).
In 11g, there has a been a change in the rules that choose between using 'direct path reads' and reads through the buffer cache for serial (i.e. non-parallel) table scans. This decision is based on the size of the table, buffer cache size and various other
statistics. Since Direct path reads are faster than scattered reads and have less impact on other processes because they avoid latches
it is likely that they will be chosen for such reads in 11g and above.
The choice can vary over time for the same tables, for example, when using Automatic Shared Memory Management (ASMM) with the buffer cache low limit set low when compared to the normal workload requirements (and usually after startup), 11g might choose to do
serial direct path read scans for large tables that do not fit in the SGA. When ASMM increases the buffer cache due to increased demand, 11g might then change to go through the buffer cache for these same large tables.
If you feel the waits are too high and you get better performance from queries by going through the cache then you can note the buffer cache and share pool requirements for a normal
workload and set the low limits of buffer cache and shared pool in spfile/pfile close to these normal workload values using the db_cache_size and shared_pool_size parameters. The danger with this approach is that you may end up with suboptimal sizes generally.
Another 'solution' would be to look at the peformance of the queries waiting for the 'direct path read' events. Perhaps they could be tuned or need to be executed in parallel? You can
use ASH reports to identify the SQL that is waiting and then use tuning advisors or manual methods to improve the performance of the queries.
Document 745216.1 Query Performance Degradation - Upgrade Related - Recommended Actions
Symptoms |
Changes |
Cause |
Solution |
References |
APPLIES TO:
Oracle Database - Enterprise Edition - Version 11.1.0.6 and laterInformation in this document applies to any platform.
SYMPTOMS
Intermittent but noticeable increase in 'direct path read' waits were observed in 11g when compared to Oracle 10g. For example, top 5 events:Top 5 Timed Foreground Events ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Avg wait % DB Event Waits Time(s) (ms) time Wait Class ------------------------------ ------------ ----------- ------ ------ ---------- DB CPU 13,916 42.1 direct path read 1,637,344 13,359 8 40.4 User I/O db file sequential read 47,132 1,111 24 3.4 User I/O DFS lock handle 301,278 1,028 3 3.1 Other db file parallel read 14,724 554 38 1.7 User I/O
Typically these will be higher at times where many serial Full Table Scans of Tables occur
Using Automatic Shared Memory Management (ASMM)
Database upgraded from 10g to 11g
CHANGES
Database upgraded from 10g to 11g
CAUSE
In 10g, serial table scans for "large" tables go through the buffer cache (by default). In 11g, there has a been a change in the rules that choose between using 'direct path reads' and reads through the buffer cache for serial (i.e. non-parallel) table scans. This decision is based on the size of the table, buffer cache size and various other
statistics. Since Direct path reads are faster than scattered reads and have less impact on other processes because they avoid latches
it is likely that they will be chosen for such reads in 11g and above.
The choice can vary over time for the same tables, for example, when using Automatic Shared Memory Management (ASMM) with the buffer cache low limit set low when compared to the normal workload requirements (and usually after startup), 11g might choose to do
serial direct path read scans for large tables that do not fit in the SGA. When ASMM increases the buffer cache due to increased demand, 11g might then change to go through the buffer cache for these same large tables.
SOLUTION
If you feel the waits are too high and you get better performance from queries by going through the cache then you can note the buffer cache and share pool requirements for a normalworkload and set the low limits of buffer cache and shared pool in spfile/pfile close to these normal workload values using the db_cache_size and shared_pool_size parameters. The danger with this approach is that you may end up with suboptimal sizes generally.
Another 'solution' would be to look at the peformance of the queries waiting for the 'direct path read' events. Perhaps they could be tuned or need to be executed in parallel? You can
use ASH reports to identify the SQL that is waiting and then use tuning advisors or manual methods to improve the performance of the queries.
Document 745216.1 Query Performance Degradation - Upgrade Related - Recommended Actions
相关文章推荐
- High "Resmgr:Cpu Quantum" Wait Events In 11g Even When Resource Manager Is Disabled (文档 ID 949033.1)
- How To Query And Change The Oracle Hidden Parameters In Oracle 10g and 11g (文档 ID 315631.1)
- Oracle 11g direct path read 等待事件的理解
- Oracle 11g全表扫描以Direct Path Read方式执行
- How to Debug root.sh Problems when Installing CRS 10.2 and 11.1 Release (Doc ID 443418.1)
- How to collect the full path name of the files in ASM diskgroups (文档 ID 888943.1)
- 11g direct path read介绍:10949 event、_small_table_threshold与_serial_direct_read
- How To Trace The Remote File Server (RFS) Process In Physical Standby Database (Doc ID 1481125.1)
- SQL -- What Tables Queries are Used to Display the Counts in the Inventory Account Periods form (INVTTGPM.fmb) (Doc ID ID 357997.1)
- How to Proceed From a Failed 10g or 11.1 Oracle Clusterware (CRS) Installation (Doc ID 239998.1)
- 【转自Oracle ACE-盖国强】Oracle 11g全表扫描以Direct Path Read方式执行
- ORA-24300 When Configuring GC Agent to monitor 11g ASM [ID 977472.1]
- How to Read Buffer Cache Advisory Section in AWR and Statspack Reports. (文档 ID 754639.1)
- How to Gather Optimizer Statistics on 10g (Doc ID 605439.1)
- 11g direct path read介绍:10949 event、_small_table_threshold与_serial_direct_read
- Incrementally Updated Backup In 10G and higher (文档 ID 303861.1)
- High Waits on 'Db File Sequential Read' Due to Table Lookup Following Index Access (文档 ID 875472.1)
- oracle wait event: Direct Path Read Temp
- Oracle 10g: Checklist To Diagnose Issues Related To Automatic Run Of Job GATHER_STATS_JOB (Doc ID 80
- How to Gather Optimizer Statistics on 11g (Doc ID 749227.1)