Deep Dive into Oracle Undo Tablespace Management in 19c

As we discussed some undo-related aspects in my previous blog post , I’ll continue the conversation here with more details about undo tablespaces and the common challenges you might encounter with them in Oracle 19c. Understanding and optimizing undo management is critical to ensuring data consistency, supporting recovery operations, and maintaining peak database performance.

What is Undo Tablespace?

Undo tablespaces are special storage spaces in Oracle databases responsible for keeping the before-images of data changed by transactions. They enable key functionalities like:

• Data Consistency

They allow queries to read consistent data, even if the data is being modified by other transactions simultaneously.

• Transaction Rollbacks

If a transaction fails or gets rolled back, the undo data helps reverse the changes.

• Flashback Features

Undo data powers Oracle Flashback technologies, allowing the retrieval of past data states or point-in-time recovery.

Components of Undo Management

1. Types of Undo Data

• Undo Records: Store the before-image of modified data.
• Undo Segments: Logical structures within the undo tablespace holding undo records.
• Undo Blocks: The smallest unit of undo data, residing within datafiles.

2. Undo Tablespace Management Modes

• Automatic Undo Management (AUM): Oracle handles undo allocation/deallocation automatically.
• Manual Undo Management: Rarely used; the DBA manages undo segments explicitly.

3. Configuration & Key Parameters

• UNDO_RETENTION: The minimum time (in seconds) Oracle tries to retain undo data.
• UNDO_TABLESPACE: Specifies the active undo tablespace.
• UNDO_MANAGEMENT: Should be set to AUTO to enable AUM.

Common Undo Issues and Solutions

1. ORA-01555: Snapshot Too Old

Issue: Occurs when a long-running query attempts to access older undo data that has already been overwritten.

Analysis:

• Undo retention might be set too low for long-running queries.
• High transaction volumes can cause undo data to wrap around too quickly.

Solution:

// Increase Undo retention
ALTER SYSTEM SET UNDO_RETENTION = 7200; -- 2 hours

// Identify Long-Running Queries:
SELECT SID, SQL_ID, LAST_CALL_ET  FROM V$SESSION WHERE LAST_CALL_ET > 3600;


// Resize Undo Tablespace:
ALTER DATABASE DATAFILE '/u01/datafile/undo/undotbs01.dbf' 

AUTOEXTEND ON NEXT 200M MAXSIZE UNLIMITED;

2. Undo Contention

Issue: Multiple sessions attempt to access or modify the same undo segments, causing contention and performance degradation.

Analysis: High concurrency or inadequate undo segments can trigger contention.

Solution:

// Monitor Undo Contention:
SELECT SID, EVENT, WAIT_TIME, SECONDS_IN_WAIT FROM V$SESSION_WAIT WHERE EVENT LIKE '%undo%';


// Increase Undo Tablespace Size:
ALTER DATABASE DATAFILE '/u01/datafile/undo/undotbs01.dbf' AUTOEXTEND ON NEXT 100M MAXSIZE UNLIMITED;


// In RAC Environments: Assign separate undo tablespaces to each instance:


ALTER SYSTEM SET UNDO_TABLESPACE = 'UNDOTBS1' SID = 'rac_inst1';
ALTER SYSTEM SET UNDO_TABLESPACE = 'UNDOTBS2' SID = 'rac_inst2';

3. Undo Data Loss

Issue: Undo data is overwritten before it’s needed, resulting in errors or inconsistent results.

Analysis: Retention period too short for workload. Large or frequent transactions exhausting available undo space.

Solution:

// Extend Retention Period:
ALTER SYSTEM SET UNDO_RETENTION = 14400; -- 4 hours

// Monitor Undo Usage:
SELECT BEGIN_TIME, END_TIME, UNDO_ENTRIES, MAXQUERYLEN, SSOLDERRCNT  FROM V$UNDOSTAT WHERE SSOLDERRCNT > 0;

4. Fragmentation in Undo Tablespace

Issue: Over time, space within the undo tablespace can become fragmented, reducing efficiency.

Solution:

// Monitor Fragmentation:
SELECT TABLESPACE_NAME, SUM(BYTES)/1024/1024 AS TOTAL_MB  FROM DBA_DATA_FILES WHERE TABLESPACE_NAME = 'UNDOTBS1';


// Reorganize the Tablespace i.e, Create a new undo tablespace and migrate:
CREATE UNDO TABLESPACE undotbs_new DATAFILE '/path/undotbs_new.dbf' SIZE 2G;
ALTER SYSTEM SET UNDO_TABLESPACE = undotbs_new;
DROP TABLESPACE undotbs_old INCLUDING CONTENTS;

5. Misconfigured Flashback Features

Issue: Incorrect Flashback settings can lead to undo-related issues, particularly during recovery or historical queries.

Solution:

// Enable Flashback Database:
ALTER DATABASE FLASHBACK ON;


// Set Flashback Retention:
ALTER SYSTEM SET DB_FLASHBACK_RETENTION_TARGET = 1440; -- 24 hours

Real-World Scenario: Troubleshooting Frequent ORA-01555 Errors

Let’s say your production environment experiences frequent ORA-01555 errors during peak hours. Here’s a step-by-step approach:

// Verify Undo Retention Setting:
SHOW PARAMETER UNDO_RETENTION;

// Monitor Undo Usage:
SELECT BEGIN_TIME, END_TIME, UNDO_BLOCKS, MAXQUERYLEN  FROM V$UNDOSTAT WHERE SSOLDERRCNT > 0;


// Optimize Long Queries:
SELECT SID, SQL_ID, LAST_CALL_ET  FROM V$SESSION WHERE LAST_CALL_ET > 3600;

If the errors persist, increase the undo tablespace size or break down large transactions into smaller, manageable batches.

Conclusion

Proper management of undo tablespaces is critical to ensuring seamless transaction processing and data recovery in Oracle 19c. Addressing issues like ORA-01555, undo contention, and fragmentation requires a combination of monitoring, configuration, and optimization strategies.

In multi-instance RAC environments, assigning separate undo tablespaces to each instance and balancing workloads are key best practices. With careful planning and proactive monitoring, you can avoid common undo-related challenges and ensure your Oracle database runs efficiently.

Stay tuned for more insights on database performance tuning, and feel free to explore the undo-related topics we discussed earlier. Managing undo tablespaces effectively is a vital skill, and mastering it will elevate your database administration expertise. Happy tuning!