Redo Log Buffer (Performance Issues)

Redo Log Buffer and Performance Issues related to redo log buffer

Performance issues related to the Redo Log Buffer in Oracle 19c primarily center on contention, insufficient sizing, and inefficient log-writing processes. Understanding these issues helps ensure better performance and reliability for transactional workloads.
Here’s a detailed breakdown:
What is the Redo Log Buffer? The redo log buffer is a memory area in Oracle that temporarily stores redo entries (records of changes made to the database). These redo entries are eventually written by the Log Writer process (LGWR) into the redo log files on disk.
Potential Performance Issues:
1. Redo Log Buffer Contention

Cause:- Multiple concurrent transactions competing for space in the redo log buffer.
- High activity on transactional systems can result in frequent waiting for redo buffer space.
Symptoms:- "log buffer space" wait events appear frequently.
- Reduced transaction throughput.
- (This item describes symptoms, not child items, so no further nested list is needed)
Resolution:- Increase the redo log buffer size (`LOG_BUFFER` parameter).
- Optimize transactions (commit less frequently in small transactions).

2. Inefficient or Delayed Writes by LGWR

Cause:- Slow disk I/O subsystem.
- Inefficient disk configuration (lack of fast SSDs or insufficient IOPS).
- Overloaded LGWR process due to excessive commit frequency or small redo log files.
Symptoms:- Increased wait events like "log file sync" or "log file parallel write".
- Users experience increased latency during commits.
- (This item describes symptoms, not child items, so no further nested list is needed)
Resolution:- Improve disk I/O subsystem performance (SSD, RAID configuration).
- Multiplex redo log files across different storage devices.
- Adjust commit frequency in applications.

3. Insufficient Redo Log Buffer Size

Cause:- Redo buffer size too small to accommodate transaction load.
- Results in increased frequency of LGWR writes to disk.
Symptoms:- Frequent "log buffer space" waits.
- Reduced transaction throughput, high CPU overhead.
- (This item describes symptoms, not child items, so no further nested list is needed)
Resolution:- Appropriately size the redo log buffer (default managed automatically in Oracle 19c, but manual tuning may be needed):
- ```
ALTER SYSTEM SET LOG_BUFFER = [appropriate_size] SCOPE=SPFILE;
```
- Reboot the instance after changes.

4. High Transaction Commit Rates

Cause:- Applications frequently issuing short transactions (frequent commits).
- High “log file sync” waits.
- Increased I/O overhead on redo logs.
Symptoms:- High “log file sync” waits. - Increased I/O overhead on redo logs.
- (This item describes symptoms, not child items, so no further nested list is needed)
Resolution:- Batch transactions and commit less frequently.
- Tune applications to use fewer commits for bulk operations.
- (This item describes resolutions, not child items, so no further nested list is needed)

Best Practices for Redo Log Buffer in Oracle 19c:- Allow Oracle Automatic Memory Management to set redo log buffer size.

Monitor frequently using:

SELECT event, total_waits, average_wait 
FROM v$system_event 
WHERE event IN ('log file sync', 'log file parallel write', 'log buffer space');

Optimize disk I/O performance (SSD or NVMe).
Regularly evaluate LGWR performance.

Conclusion: Redo log buffer performance issues in Oracle 19c are usually due to contention, inefficient buffer sizing, excessive commit frequency, or disk I/O bottlenecks. Addressing these through buffer sizing, application tuning, and storage optimization significantly improves Oracle database performance.
The redo log buffer contains the Oracle redo logs that hold previous versions of Oracle transactions and are used to roll-forward the database in cases of disk failure. When a database is in ARCHIVELOG mode, Oracle will write these redo logs onto a disk file with the ARCH background process. Regardless of the ARCHIVELOG mode, Oracle must still manage the redo images and write them to the online redo logs, via the redo log buffer.

Oracle System Global Area (SGA)

The provided diagram is a simplified representation of the Oracle System Global Area (SGA), highlighting key components within it, with particular focus on the Redo Log Buffer. Here's a detailed explanation of each component shown:

**Shared Pool** (Left-hand side box)

**Session Memory:** Stores session-specific information for each connected user, including user-specific session variables and context data.

**Library Cache:** Stores parsed SQL statements, execution plans, and PL/SQL program units to avoid redundant parsing and compilation.

**Dictionary Cache**: Holds frequently accessed data dictionary information such as object definitions, privileges, and users.

**Redo Log Buffer (highlighted in orange with red outline)**:

Temporarily stores changes made to the database.

Changes are recorded here before being written to redo log files on disk.

Essential for recovery operations, ensuring transaction durability and consistency.

**Database Buffer Cache**:

Caches copies of data blocks from disk to minimize disk I/O.

Provides quick access to frequently used data blocks.

In a typical database operation sequence:

When a database transaction occurs, the change information (redo entries) first goes into the Redo Log Buffer.

Eventually, these entries are flushed (written) by the Log Writer process (LGWR) to the Redo Log Files, ensuring durability and recoverability of database transactions.

The redo information is critical for recovery operations, database rollbacks, and to maintain ACID properties.

The emphasis on the Redo Log Buffer underscores its importance in managing database changes, recovery, and transaction consistency within Oracle's database architecture. Oracle redo log buffer is one of 3 components in the SGA

Redo logs and Performance Problems

There are several areas where redo log processing can cause a performance problem:

When the redo logs switch too frequently (> 2 per hour)
When the database experiences a high value for redo log space requests
When the archived redo log file system becomes full.

Since the redo log buffer is used to cache update, insert or delete activity, high update activity would be associated with redo log buffer activity. A high rate of redo log switching or high redo log space requests always indicates a problem. The remedy is to increase the log_buffer init.ora parameter.

Detecting high values for redo log space Requests
While we will explore these issues in detail in a later module, let us look at a simple query that can detect a high redo log space request. Note that it queries the V$SYSSTAT view. When you see high redo log space requests you can increase the init.ora parameter called log_buffer.
To increase the size of the log buffer, you increase the log_buffer parameter in your init.ora file and re-start the Oracle instance. This parameter determines the amount of memory to allocate for Oracle's redo log buffers. If there is a high amount of update activity, you should allocate more space in the log_buffer.
```
prompt
prompt         =========================
prompt         REDO LOG BUFFER
prompt         =========================
prompt (should be near 0, else increase size of LOG_BUFFER in init.ora)
prompt
set heading off
column value format 999,999,999
select substr(name,1,30),
       value
from v$sysstat where name = 'redo log space requests';
Here is what the output from this query might look like:
=========================
REDO LOG BUFFER
=========================
(should be near 0, else increase size of LOG_BUFFER in init.ora)
redo log space requests                   0
```

The next lesson concludes this module.

Redo Log - Exercise

Before you go on, click the Exercise link below to check your knowledge of basic SGA tuning concepts.
Redo Log - Exercise

Lesson 8	Tuning considerations of the redo log buffer
Objective	Describe the performance issues related to the redo log buffer.