How do I resolve log file sync wait event?

If the waits are because of slow I/O, then try the following: Reduce other I/O activity on the disks containing the redo logs, or use dedicated disks. Try to reduce resource contention. Check the number of transactions (commits + rollbacks) each second, from V$SYSSTAT.

What is redo log sync?

Log file sync (%) When a user session COMMITs (or rolls back), the sessions redo information needs to be flushed to the redo log file. The user session will post the LGWR to write all redo required from the log buffer to the redo log file. When the LGWR has finished it will post the user session.

What is log file parallel write wait event?

The log file parallel write Oracle metric indicates the process is waiting for blocks to be written to all online redo log members in one group. LGWR is typically the only process to see this wait event. It will wait until all blocks have been written to all members.

How do you improve redo log writing performance?

The steps for tuning redo log performance are straightforward:

1. Determine the optimal sizing of the log_buffer.
2. Size online redo logs to control the frequency of log switches and minimize system waits.
3. Optimize the redo log disk to prevent bottlenecks.

What is Lgwr?

LGWR (LoG WRiter) is an Oracle background process created when you start a database instance. The LGWR writes the redo log buffers to the on-line redo log files. If the on-line redo log files are mirrored, all the members of the group will be written out simultaneously.

What is direct path read in Oracle?

A direct read is a physical I/O from a data file that bypasses the buffer cache and reads the data block directly into process-private memory. If asynchronous I/O is supported (and in use), then Oracle can submit I/O requests and continue processing.

What is DB file sequential read in Oracle?

“The db file sequential read Oracle metric event signifies that the user process is reading buffers into the SGA buffer cache and is waiting for a physical I/O call to return. This call differs from a scattered read, because a sequential read is reading data into contiguous memory space.

What is library cache lock in Oracle?

Description. The library cache lock controls the concurrency between clients of the library cache by acquiring a lock on the object handle so that one client can prevent other clients from accessing the same object or the client can maintain a dependency for a long time (no other client can change the object).

Why it is required to store redo logs in faster disks?

The idea of storing redo log in the RAM is to reduce disk I/O. Of-course, when a transaction commits, the redo log buffer must be flushed to disk, otherwise the recovery for that commit could not be guaranteed. It is LGWR (Log Writer) process which is responsible for redo log buffer flushing into disk .

How do you find the frequency of a redo log switch?

Ideally redo log switch frequency should be 4-5 log switches per hour. If there is frequent log switches then check the size of redo logs and increase accordingly. V$LOG_HISTORY is used to check history of redo log generation in Oracle. V$LOGFILE is used to check the file name and location of redo logs.

What is Smon and Pmon in Oracle?

SMON -> performs recovery, when instance fails and, PMON -> performs recovery, when user process fails.

What does the Lgwr process do?

The log writer process (LGWR) is responsible for redo log buffer management—writing the redo log buffer to a redo log file on disk. LGWR writes all redo entries that have been copied into the buffer since the last time it wrote.

What causes redo log file sync wait time?

Log file sync waits occur when the LGWR process is unable to complete writes fast enough Redo log activity increases as a function of system activity, and high log file sync waits may occur during periods of high DML activity. – Slow disk I/O: Segregating the redo log file onto separate disk spindles can reduce log file sync waits.

What is this ‘log file sync’ note for?

The purpose of this note is to help troubleshoot issues where there are significant waits for the event ‘log file sync’. To view full details, sign in with your My Oracle Support account. Don’t have a My Oracle Support account?

How does Oracle Process log file sync wait?

Oracle guru Steve Adams notes details on how Oracle processes log file sync waits: “Before writing a batch of database blocks, DBWn finds the highest high redo block address that needs to be synced before the batch can be written.

How can I speed up the log sync in AWR?

Moving the online redo logs to fast SSD storage and increasing the log_buffer size above 10 megabytes (It is automatically set in 11g and beyond). If I/O is slow (timings in AWR or STATSPACK reports > 15 ms), then the only solution for log file sync waits is to improve I/O bandwidth.