Wait Events: "Log File Sync and Log File Parallel Write"

In OLTP environments, one of the biggest performance problems is the excessive wait events for recordings Online Redo Log Files. Every time that Oracle Database waits for recordings in these files can observe events "Log File Sync 'and' Log File Parallel Write".

 


What is the expected event 'log file sync'?

 

When a session of a user or application executes a COMMIT, the information stored in memory Redo (LOG_BUFFER) are discharged in the Online Redo Log Files to make permanent changes made in the database. This process is executed by LOG WRITER (LGWR). When the LGWR process ends this recording, a warning is sent to the session stating that the COMMIT process is 100% complete.
The time between the user session / application LGWR request the recording of redos Memory (LOG_BUFFER) for Online Redo Log Files and LGWR inform the meeting that the process was completed is where the event occurs 'log file
sync'.

How to identify the event 'log file sync'?

 

To analyze and identify the event 'log file sync' can use the information to:
- Reports of a period AWR timely collection.
- Trace files of LGWR. These files can show many events waiting on 'log file parallel wait'

Events which occur Waiting 'log file sync'?

 

Events waiting to 'log file sync' can happen at any stage between the user session / application LGWR request the recording of redos Memory (LOG_BUFFER) for Online Redo Log Files and LGWR inform the session that the process was completed.

Performance of LGWR

 

The main question is "Is there any problem for Recording Performance in Online Redo Logs"?
The steps below can help you find performance problems in the Log Writer (LGWR).

1 - Comparing the average waiting time for 'log file sync' with the average waiting time for 'log file parallel write'

 

The event hopes to 'log file parallel write' occurs while LGWR is writing redo information on Online Redo Log Files. The Total time of this event is the total recording time in disk files.
Analyzing this event together with the event 'log file sync' can identify the total time consumption and I / O completion of a cycle COMMIT.

Analyzing the information in an AWR report:

 


In the above example, we can identify a very time-consuming and I / O (ms) for both events 'log file sync' and 'log file parallel write'.
The proportion of time between 'log file sync' and 'log file parallel write' suggests a problem of I / O (Waiting recording redo from memory to disk).

If the time spent in waiting event 'log file parallel write' exceeds 20ms, the likelihood of a problem of I / O is great.

 

Recommendations
- Check the filesystems where are the Redo Logs, and check possbilidade to optimize the performance of I / O through OS
- Do not use RAID 5 for filesystems / disks where are the Redo Logs.
- Ensure that there are other processes that can be recording information on the same disks where are the Redo Logs. Make sure that the disk has enough throughput for competition of these processes, or change the Redo Logs Filesystem.
- Check if the parameter is not too large LOG_BUFFER. The higher the LOG_BUFFER the longer it takes for the LGWR to write redo the changes to disk.

2 - Check the Traces of the Log Writer (LGWR)

 

Even if the average waiting time of the event 'log file parallel write' is normal, there may be peaks where the writing of the redo logs is high in influence and then the event 'log file sync'.
From version 10.2.0.4 are generated alert messages in the LGWR trace file when a process LGWR writing takes longer than 500ms. This is a high threshold, but it is a way of identifying problems I / O. Messages are other similar to:

Recommendations

 

- Check what else is running at the moment to identify gaps recording LGWR.

3 - Check if the size of Redo logs is Enough
An operation 'log file sync' is run every time there is a switch of logfiles to ensure that all information in redos were recorded before the next log starts. As a standard recommendation is that a log switch occurs every 15 or 20 minutes. If the log switches occur in a shorter time, more often, then more wait events 'log file sync' will occur, which means a longer waiting time between sessions.

- You can check the alert.log to check the switch logfile:

 


In the above switches can be checked every 2 to 4 minutes, which is 5x more frequently than desirable.
- Another option is to check the average Log Switchs through AWR report:
In the example above 29.98 log switches occur every hour, which means about 2 log switches per minute. This is a value much higher than recommended (15 to 20 switches per minute).
Recommendations
- Increasing the size of Redo Log files.

4 - Excessive number of commits by Application

 

In this case the question is what to do: "The application is performing many commits?"
If so, there may be performance issues, since the redo information are unloaded from memory (LOG_BUFFER) redo logs to disk.
A simple way to identify this scenario is when the average wait for the event 'log file sync' is much larger than average wait event "log file parallel write ', this shows that the longer the waiting is not for recording Disco redo logs. The high CPU time can also show restraint caused by excessive commits.
Another scenario is still where the average wait event "log file sync 'is low but the number of waits is high.
One can compare the average 'User commits' with 'User calls':

The above example could identify 5.76 'user calls' commit by what is considered 5x greater than recommended (1).

 

Recommendations:
- If multiple commits in the application in a short time interval, or small call commits, it is recommended to execute a block of larger changes before executing a commit. Ateração This should be done through the application.
- Enabling Asynchronous Commit. (To learn how to enable this option, see the article:Changing behavior with transactions in Oracle Database Asynchronous... )

Sources:

How to Minimise Waits for 'Log File Sync'? [MOS ID 857576.1]
Troubleshooting: log file sync 'Waits [MOS ID 1376916.1]

Victor

Views: 7843

Comment by Madhava Verma Dantuluri on February 4, 2014 at 5:47am

Excellent article, i learnt something new today.

Comment by Victor Armbrust on February 4, 2014 at 10:03am

Thanks Madhava. Pleasure to help. =D

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