Oracle databases handle large objects (LOBs) such as CLOB, BLOB, NCLOB, and SecureFiles efficiently, but when storage management is not planned correctly, DBAs often face one of the most frustrating errors:
ORA-01691: unable to extend lob segment
This error usually appears suddenly—during inserts, updates, data loads, or application operations—and can immediately impact production workloads.
In this guide, we’ll break down:
- What ORA-01691 really means
- Why it happens
- How to identify the exact cause
- Step-by-step solutions
- Long-term best practices to prevent it
This is written from a real DBA perspective, not just documentation theory.
What Does ORA-01691 Mean?
ORA-01691 occurs when Oracle is unable to allocate additional space for a LOB segment inside its tablespace.
LOB segments grow differently from normal table segments. They often:
- Grow rapidly
- Use large extents
- Fragment faster
- Consume space unexpectedly
When Oracle tries to extend the LOB segment but cannot find free space, it throws ORA-01691.
Typical Error Message
ORA-01691: unable to extend lob segment <owner>.<lob_segment_name> in tablespace <tablespace_name>
Example:
ORA-01691: unable to extend lob segment HR.SYS_LOB0000123456C00002$$ in tablespace USERS
Common Causes of ORA-01691
Let’s look at the real reasons DBAs encounter this error.
1. Tablespace Is Out of Free Space
The most common cause.
- Tablespace has no free extents
- Datafiles reached max size
- Autoextend is disabled
LOBs often require large contiguous extents, so even if some space exists, it may not be usable.
2. Autoextend Is Disabled or MAXSIZE Reached
Even if autoextend is enabled, the datafile may have:
- Reached
MAXSIZE - Hit filesystem or ASM diskgroup limits
3. Fragmented Tablespace
LOB segments need large continuous free extents.
If the tablespace is fragmented:
- Free space exists
- But not in large enough chunks
- Oracle fails to extend the LOB segment
4. LOB Segment Placed in Wrong Tablespace
Many applications:
- Store LOBs in default tablespaces (like USERS)
- Share space with normal tables and indexes
This is a design mistake and often leads to ORA-01691.
5. SecureFile or BasicFile LOB Growth
SecureFiles improve performance but:
- Can grow aggressively
- Consume space faster if compression/deduplication is not enabled
6. Heavy DML or Batch Loads
Bulk inserts, ETL jobs, or application uploads (PDFs, images, JSON, XML) can suddenly exhaust LOB space.
How to Identify the Affected LOB Segment
First, identify which LOB segment is failing.
Find LOB Segment Details
SELECT owner, table_name, column_name, segment_name, tablespace_name
FROM dba_lobs
WHERE segment_name = 'SYS_LOB0000123456C00002$$';
This tells you:
- Which table
- Which column
- Which tablespace
Check Tablespace Free Space
SELECT tablespace_name,
ROUND(SUM(bytes)/1024/1024) AS free_mb
FROM dba_free_space
WHERE tablespace_name = 'USERS'
GROUP BY tablespace_name;
If free space is low—or fragmented—you’ve found the problem.
Immediate Fixes for ORA-01691
Solution 1: Add Space to the Tablespace (Best Practice)
If using filesystem:
ALTER TABLESPACE users
ADD DATAFILE '/u01/oradata/DB/users02.dbf'
SIZE 10G AUTOEXTEND ON NEXT 1G MAXSIZE UNLIMITED;
If using ASM:
ALTER TABLESPACE users
ADD DATAFILE '+DATA'
SIZE 10G AUTOEXTEND ON NEXT 1G;
Solution 2: Enable Autoextend
ALTER DATABASE DATAFILE '/u01/oradata/DB/users01.dbf'
AUTOEXTEND ON NEXT 1G MAXSIZE UNLIMITED;
Always verify MAXSIZE.
Solution 3: Move LOB to a New Tablespace
Create a dedicated LOB tablespace:
CREATE TABLESPACE lob_ts
DATAFILE '+DATA'
SIZE 20G AUTOEXTEND ON NEXT 2G;
Move the LOB:
ALTER TABLE hr.documents
MOVE LOB (doc_content)
STORE AS (TABLESPACE lob_ts);
This is the cleanest long-term solution.
Solution 4: Shrink or Rebuild LOB Segment (If Possible)
If SecureFiles are enabled:
ALTER TABLE hr.documents
MODIFY LOB (doc_content) (SHRINK SPACE);
This helps reclaim unused space.
Solution 5: Convert BASICFILE to SECUREFILE
SecureFiles manage space better.
ALTER TABLE hr.documents
MOVE LOB (doc_content)
STORE AS SECUREFILE;
Optional enhancements:
STORE AS SECUREFILE (
COMPRESS HIGH
DEDUPLICATE
);
Long-Term DBA Best Practices to Prevent ORA-01691
1. Always Separate LOB Tablespaces
Never store LOBs in:
- USERS
- DATA
- Index tablespaces
Use dedicated tablespaces.
2. Monitor LOB Growth Proactively
SELECT segment_name,
tablespace_name,
ROUND(bytes/1024/1024) AS size_mb
FROM dba_segments
WHERE segment_type = 'LOBSEGMENT'
ORDER BY size_mb DESC;
3. Use SecureFiles by Default
SecureFiles offer:
- Better space management
- Compression
- Deduplication
- Faster access
4. Enable Autoextend with Sensible Limits
Avoid unlimited growth without monitoring.
5. Plan for Application Upload Patterns
If the application handles:
- Files
- Images
- JSON
- XML
- Logs
LOB growth must be part of capacity planning.
Why ORA-01691 Is a Design Problem, Not Just a Space Issue
Most ORA-01691 errors are not random failures—they are architecture flaws:
- Poor tablespace separation
- No LOB capacity planning
- Ignoring SecureFiles features
- No proactive monitoring
Fixing the root cause prevents repeated outages.
Conclusion
ORA-01691: unable to extend lob segment is one of the most common and avoidable Oracle storage errors.
To fix it properly:
- Don’t just add space
- Identify the LOB
- Move it to the right tablespace
- Use SecureFiles
- Monitor growth continuously
A well-designed LOB strategy ensures your database remains stable, scalable, and production-ready.
