This has been very helpful to us. If the average wait time is low, but the number of waits is high, then the application might be committing after every row, rather than batching COMMITs. Wayne from Texas Tom, AS always, great job.
Chapter 7 has more details on the log file sync wait event. Make sure the controller that services the mount point is not overloaded.
Slow Write on Hard Disk: OS Watcher will give you a fair idea about over all load on the system. In this case, it is critical that nothing else is sharing the mount point of the redo log files. Unfortunately, you cannot spawn more than one LGWR process. The overall wait time for "log file sync" can be broken down into several components.
Performance troubleshooting as exact science High log file sync waits? During log file switching process LGWR writes all log buffer data into redo log files and goes into log file sync wait event.
Whenever user issue a commit statement, LGWR start writing data from log buffer cache to redo log files. Every time user execute commit command LGWR write log buffer data into redo log files which initiate log file sync wait event. This is because even after the foreground has been posted it may take some time for the OS to schedule it to run.
Few can solve log file sync wait Event easily but for other this could be a pain. A reader meant to select 5 stars August 21, - 2: Although commits can be "piggybacked" by Oracle, reducing the overall number of commits by batching transactions can be very beneficial. The surplus time is CPU activity and is most commonly contention caused by over committing.
That is usually the manager doing that because they see it is cheaper and therefore must be "better": Tune the control and log devices.
Each commit has to be confirmed to make sure the relevant REDO is on disk. This event, normally, can be ignored. Do not put redo logs on RAID 5.
Each commit has to have it confirmed that the relevant REDO is on disk. In this case, modify the application to commit at a lower frequency.
In below AWR report snapshot Total time wait for Log file sync isand out of it 93, times it was waiting for log file parallel write wait event which is quite high. Keep in mind that as always with bottlenecks, removing one can cause another someplace else, so a realistic test on a production-like environment before going on production is important.
There could also be many short sessions that log in to the database, perform a quick DML operation, and log out. Do not put redo logs on RAID 5. If you are seeing frequenct log file switch consider resizing redo log file size.
Excessive Commits from Application: The vast majority of DB time is spent on log file sync, so our results are not polluted with irrelevant waits. A possible and rather common side effect is a dramatic degradation in log file sync waits which outweighs benefits from reduced latching.
A lower commit frequency at the expense of higher rollback segment usage can also provide some relief. Database administrator has to compare and Analyze these AWR repots.
The "log file sync" wait event may be broken down into the following components: Although commits can be piggybacked by Oracle, reducing the overall number of commits by batching transactions can have a very beneficial effect. To verify frequency of log file switching check alert log file which keep record of each log file switch.
Next, collect at least two AWR Reports one for good time and another at time of high log file sync wait event. See Metalink Note The cure is the same as for the db file parallel write waits. If there are lots of short duration transactions, see if it is possible to group transactions together so there are fewer distinct COMMIT operations.
Ask you network admin to look into and find out root cause of the issue.– broad tails in log file parallel write histograms (and consequently high values of log file sync to log file parallel write average wait ratio, and higher) are strong indicators of log.
Jun 17, · The Oracle "log file sync" wait event is triggered when a user session issues a commit (or a rollback). The user session will signal or post the LGWR to write the log buffer to the redo log file. When the LGWR has finished writing, it will post the user session.
I have a task which reads a large file line by line, does some logic with it, and returns a string I need to write to a file. The order of the output does not matter. However, when I try the code below, it stops/get really slow after reading k lines of my file.
Oracle log file parallel writes Oracle db file sequential scattered read disk I/O speed tuning The popular Ion tool is the easiest way to analyze Oracle log file behavior over time (above plot of log file parallel writes), and Ion allows you to spot hidden redo log performance trends.
A bigger log buffer and FAR FAR FAR fewer commits (you are probably committing way way too much -- that is causing extra control file writes and log waits - the only time you want for logs; when you commit or the redo log buffer is totally full). Query the V$SYSTEM_EVENT view as follows to discover the LGWR average wait time (log file parallel write event).
An average wait time of 10ms (1 centisecond) or less is generally acceptable. A higher system I/O throughput can improve the average wait time of .Download