AWR / Statspack Analysis

Problem : Performance and High CPU

Solution / Interpretation : The TOP Event log file sync This wait event is seen in most of the high transactional databases where more frequent commits happens. Let us look deep into the concept and then see how we can reduce that, definitely we cannot avoid it.

At commit time, process creates a redo record [ containing commit op codes] and copies that redo record in to log buffer. Then that process signals LGWR to write contents of log buffer. LGWR writes from log buffer to log file and signals user process back completing a commit. Commit is considered successful after LGWR write is successful.

Commit is not complete until LGWR writes log buffers including commit redo recodes to log files. In a nutshell, after posting LGWR to write, user or background processes waits for LGWR to signal back with 1 sec timeout. User process charges this wait time as ‘log file sync’ event.


LGWR is unable to complete writes fast enough for one of the below reasons..

1)Disk I/O performance to log files is not good enough.

2)LGWR is starving for CPU resource. If the server is very busy, then LGWR can starve for CPU too. This will lead to slower response from LGWR, increasing ‘log file sync’ waits.

3)LGWR is unable to complete writes fast enough due to file system or unix buffer cache limitations.

4)LGWR is unable to post the processes fast enough, due to excessive commits. It is quite possible that there is no starvation for cpu or memory and I/O performance is decent enough. Still, if there are excessive commits, then LGWR has to perform many writes/semctl calls and this can increase ‘log file sync’ waits. This can also result in sharp increase in redo wastage’ statistics’.

This event is also the major contributor for overall CPU usage.

In the above scenario, we cannot avoid the COMMITS as the application requires it. What i have done is reduced the CPU contention so that LGWR does not wait for the lack of CPU.

The other aspect is to see where these online redo log files are created. We have made raw devices for these files so that the writing happened fast.

Also there is a misconception that increasing the redo log buffer will solve the problem , but this is not the case as if you the see fundamentals of log buffer flush is for every commit or when 1/3 is full or 1 MB of redo is generated. In this case for every commit there is redo buffer flush happenes so no need to have the redo buffer more than 3MB.

Also try to place the major hot tables and indexes separately on to multiple disks to increase the disk i/o.

Team ORAKHOJ
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AWR / Statspack Analysis

Problem : Performance and CPU





Solution : The 3rd TOP 5 Wait Event 'log file switch' indicates there is a problem with Checkpoint and Switching. It directly relates to a problem with the size of the online redo log files. According to the statpack we found that there are 70 + switches perfoemd by the database per hour and also a Delete Statement was found as the TOP CPU Consuming Statement, It clearly states that there is a bottleneck during the delete operation as much redo will be generated and because of small online redo log files, switched are happening frequently which in turn making the delete statement wait for log switch to perform. Also the delete statement is using the Index which is the cuase for the first 'db file sequential read ' wait event.

Happy Tuning
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Statspack/ AWR Analysis

Oracle Release 10.2.0.3
Problem CPU and Performance

Top 5 Wait Events

You can find 'db file sequential read' and 'read by other session' apart from CPU as TOP 5Events. Based on the 'db file sequential read' it is evident that the system is using most of the indexes.

Once we go to the other wait event 'read by other session' which is nothing but buffer busy waits where information is requested from the database, Oracle will first read the data from disk into the database buffer cache. If two or more sessions request the same information, the first session will read the data into the buffer cache while other sessions wait; e.g. many sessions scanning the same index or performing full-table scans on the same table. Tuning this issue is a matter of finding and eliminating this contention.

So to figure which object is causing the contention, you can look at which tablespace/datafile has got buffer waits and which object is falling under the Segments by Physical Reads and Logical Reads. Based out which we have identified an index which is the cause for this contention. The same has been rebuild on to a different tablespace to reduce the contention.

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Statspack/ AWR Analysis

Database Version = 10.2.0.3



Problem : High CPU consumption.

Solution : Based on the above top 5 events one can cleary see that db file scattered read is high event after the CPU, which means that the application is performing full tablescans. Based on this one has to look at SQL ordered by CPU session of the report to find out which queries on performing full table scan. Based on the index recommendations the above problem has been sorted out.

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