- 20 Nov, 2025
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PostgreSQL Free Space Management
PostgreSQL Free Space Management: How PostgreSQL Reclaims & Tracks Storage Internally
PostgreSQL is known for its robust storage architecture, MVCC implementation, and strong performance. However, one area many DBAs and developers overlook is free space management—how PostgreSQL identifies, tracks, and reuses space created by updates/deletes.
Since PostgreSQL doesn’t overwrite row data in place (due to MVCC), deleted and updated tuples become dead tuples, generating internal fragmentation. Efficient free space management ensures that PostgreSQL reuses this space instead of endlessly consuming disk storage.
Let’s dive into how PostgreSQL handles this.
Why Free Space Becomes a Concern in PostgreSQL?
PostgreSQL uses MVCC (Multi-Version Concurrency Control), meaning:
-
An
UPDATEcreates a new version of a row -
The old version remains until vacuum removes it
-
DELETEmarks rows as dead, not removed instantly
Over time this creates:
| Process | Effect |
|---|---|
| Frequent UPDATE/DELETE | Dead tuples accumulate |
| No VACUUM | Table grows on disk unnecessarily |
| Fragmentation | Query performance degrades |
Thus, PostgreSQL must track where reusable space exists—this is where FSM comes in.
What is FSM (Free Space Map)?
The Free Space Map (FSM) is a PostgreSQL internal data structure that tracks how much free space exists on each data page of a table or index.
FSM helps PostgreSQL:
✔ Decide where to insert new tuples
✔ Avoid creating unnecessary new pages
✔ Reuse empty space efficiently
✔ Reduce table bloat
FSM improves performance by preventing excessive disk writes.
Where is FSM Stored?
Each table and index has a separate FSM file:
For example, if a table’s relfilenode is 16384, its FSM file is:
Use oid or relfilenode queries to find it:
How PostgreSQL Updates FSM
FSM is updated automatically during:
-
Vacuum
-
Page splits
-
Tuple removal
-
Bulk deletes
You can also manually trigger FSM updates by running:
VACUUM FULL physically rewrites the table, while normal VACUUM only marks space as reusable.
FSM vs Visibility Map (VM)
Many confuse FSM with Visibility Map. They track different information:
| Feature | FSM | VM |
|---|---|---|
| Stores | Free space on pages | Whether pages need vacuum |
| Focus | Reuse of storage | Skipping unnecessary vacuum scans |
| File type | _fsm |
_vm |
| Helps With | Inserts, reducing bloat | Faster VACUUM, index-only scans |
Both work together to maintain healthy storage.
Monitoring Free Space & Bloat
Check table bloat using extension:
Useful columns:
| Column | Meaning |
|---|---|
| dead_tuple_percent | How much space is wasted |
| free_percent | Free space in table |
| live_tuple_percent | Space actually used |
Best Practices for Free Space Management
| Practice | Purpose |
|---|---|
| Enable autovacuum | Continuous space reclamation |
| Tune autovacuum thresholds | Avoid delayed vacuuming |
Use pg_repack instead of VACUUM FULL for production |
Non-blocking table compaction |
| Partition high-churn tables | Reduces bloat impact |
Monitor dead tuples via pg_stat_all_tables |
Detect early fragmentation |
Example tuning:
Summary
| Concept | Description |
|---|---|
| FSM | Tracks free space for efficient inserts |
| Dead Tuples | Created from MVCC updates/deletes |
| VACUUM | Removes dead tuples and updates FSM |
| VM | Tracks visibility, reduces vacuum overhead |
| Goal | Minimize bloat & optimize storage usage |
Effective Free Space Management ensures your PostgreSQL instance performs efficiently and controls unnecessary storage growth—especially in transactional databases with heavy updates.
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