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Understanding WAL Files in PostgreSQL – For Oracle DBAs

What is WAL?

WAL (Write-Ahead Logging) ensures data durability and crash recovery. Every change is first written to WAL before being flushed to data files. 

1. WAL Location: WAL files are stored in $PGDATA/pg_wal/.
2. Fixed Size: Each WAL segment is pre-allocated with a fixed size (typically 16MB), set at cluster initialization (initdb).
3. Writing & Switching: WAL segments are written sequentially. PostgreSQL switches to the next segment when the current one is full or a manual switch is triggered (e.g., via pg_switch_wal()).
4. WAL and Archive File Size: When archived, the exact same 16MB WAL segment file is copied as raw binary to the archive destination. Therefore, archive files always match the WAL segment size.
5. Archiving: If archive_mode = on, PostgreSQL uses the archive_command to copy completed WAL segments to the archive destination (e.g., /pgArch/...). Only full segments are archived.
6. Recycling: If archiving is disabled, PostgreSQL reuses old WAL segments once they’re no longer needed for recovery or replication.

WAL Files = Redo Log Files
WAL Archives = Archive Log Files

PostgreSQL vs Oracle Comparison

Aspect	Oracle	PostgreSQL
Redo Log Files	Called Redo Log Files	Called WAL Files
Archive Log Files	Called Archive Log Files	Called WAL Archives
Redo Log File Size	~200MB (default)	16MB (default and fixed)
Redo Log Files Count	Minimum 2 or more redo log groups (Best practice: 3)	No fixed number Controlled by: min_wal_size / wal_segment_size Example: 80MB / 16MB = 5 WAL files
Archive Log File Size	~200MB (same as redo log file size)	16MB (same as WAL file size)
Log File Storage	Redo log files stored in redo log groups	WAL files stored in `pg_wal` directory
Archiving Mechanism	Archive logs created if archiving is enabled	Archived via `archive_command` if `archive_mode = on`
Overwrite Behavior	Redo logs are overwritten when full (if archive not enabled)	WAL segments are recycled/overwritten when full (if `archive_mode` is off)
Force Switch Behavior	ALTER SYSTEM SWITCH LOGFILE;	SELECT pg_switch_wal();

PostgreSQL WAL Configuration Parameters

Parameter	Meaning	Default
`wal_segment_size`	Size of each WAL file	16MB
`min_wal_size`	Minimum total WAL size retained	80MB (5 WAL files) Common in production: 8GB
`max_wal_size`	Max WAL before triggering a checkpoint	1GB Common in production: 16GB

Check Configuration

[postgres@lxicbpgdsgv01 ~]$ psql -c "SHOW wal_segment_size;"
wal_segment_size
------------------
16MB  <----- Each redo log file size
(1 row)

[postgres@lxicbpgdsgv01 ~]$


[postgres@lxicbpgdsgv01 ~]$ psql -c "SHOW min_wal_size;"
 min_wal_size
--------------
 80MB <---- 80MB/16MB = 5 <--- Total 5 redo log files with each 16MB
(1 row)

[postgres@lxicbpgdsgv01 ~]$
[postgres@lxicbpgdsgv01 ~]$ psql -c "SHOW max_wal_size;"
 max_wal_size
--------------
 1GB  <---- 1024MB
(1 row)

[postgres@lxicbpgdsgv01 ~]$

View WAL files (Redo log files)

postgres=# SELECT current_setting('data_directory') || '/pg_wal' AS wal_directory;
        wal_directory
-----------------------------
 /pgData/pgsql17/data/pg_wal  <---- Redo log file directory
(1 row)

postgres=# 
[postgres@lxicbpgdsgv01 ~]$ ls -lh /pgData/pgsql17/data/pg_wal | grep -E '^-.* [0-9]+ .* [0-9A-F]{24}$'
-rw-------. 1 postgres postgres 16M Oct 8 01:33 000000010000000000000005
-rw-------. 1 postgres postgres 16M Oct 8 01:28 000000010000000000000006
-rw-------. 1 postgres postgres 16M Oct 8 01:28 000000010000000000000007
[postgres@lxicbpgdsgv01 ~]$

min_wal_size = 80MB means PostgreSQL will try to keep at least 5 WAL segment files on disk (since 80MB ÷ 16MB = 5 files).

However, in our case, only 3 WAL files are currently present. This is likely because the system is under low load in the testing environment, so fewer WAL files are needed at the moment.

---- Lets Generate Load

postgres=# -- Create test table
DROP TABLE IF EXISTS wal_test;
CREATE TABLE wal_test (
    id serial PRIMARY KEY,
    data text
);

-- Generate WAL traffic
DO $$
DECLARE
    i integer;
BEGIN
    FOR i IN 1..50 LOOP
        -- INSERT: 10,000 rows
        INSERT INTO wal_test (data)
        SELECT repeat('wal_test_data_', 50)
        FROM generate_series(1, 10000);

        -- UPDATE: 5,000 rows using CTE with LIMIT
        WITH to_update AS (
            SELECT id FROM wal_test WHERE id % 2 = 0 LIMIT 5000
        )
        UPDATE wal_test
        SET data = data || '_updated'
        WHERE id IN (SELECT id FROM to_update);

        -- DELETE: 5,000 rows using CTE with LIMIT
        WITH to_delete AS (
            SELECT id FROM wal_test WHERE id % 3 = 0 LIMIT 5000
        )
        DELETE FROM wal_test
        WHERE id IN (SELECT id FROM to_delete);

        -- Commit to flush WAL
        COMMIT;

        -- Optional pause to slow down the loop
        PERFORM pg_sleep(0.5);
    END LOOP;
END$$;
DROP TABLE
CREATE TABLE
DO
postgres=# exit
[postgres@lxicbpgdsgv01 ~]$
[postgres@lxicbpgdsgv01 ~]$ 
ls -lrth /pgData/pgsql17/data/pg_wal
total 752M
drwx------. 2 postgres postgres    6 Sep 30 21:50 summaries
-rw-------. 1 postgres postgres  16M Oct  8 16:34 000000010000000000000009
-rw-------. 1 postgres postgres  16M Oct  8 16:34 00000001000000000000000A
-rw-------. 1 postgres postgres  16M Oct  8 16:34 00000001000000000000000B
-rw-------. 1 postgres postgres  16M Oct  8 16:34 00000001000000000000000C
-rw-------. 1 postgres postgres  16M Oct  8 16:34 00000001000000000000000D
-rw-------. 1 postgres postgres  16M Oct  8 16:34 00000001000000000000000E
-rw-------. 1 postgres postgres  16M Oct  8 16:34 00000001000000000000000F
-rw-------. 1 postgres postgres  16M Oct  8 16:34 000000010000000000000010
-rw-------. 1 postgres postgres  16M Oct  8 16:35 000000010000000000000011
-rw-------. 1 postgres postgres  16M Oct  8 16:35 000000010000000000000012
-rw-------. 1 postgres postgres  16M Oct  8 16:35 000000010000000000000013
-rw-------. 1 postgres postgres  16M Oct  8 16:35 000000010000000000000014
-rw-------. 1 postgres postgres  16M Oct  8 16:35 000000010000000000000015
-rw-------. 1 postgres postgres  16M Oct  8 16:35 000000010000000000000016
-rw-------. 1 postgres postgres  16M Oct  8 16:35 000000010000000000000017
-rw-------. 1 postgres postgres  16M Oct  8 16:35 000000010000000000000018
-rw-------. 1 postgres postgres  16M Oct  8 16:35 000000010000000000000019
-rw-------. 1 postgres postgres  16M Oct  8 16:35 00000001000000000000001A
-rw-------. 1 postgres postgres  16M Oct  8 16:35 00000001000000000000001B
-rw-------. 1 postgres postgres  16M Oct  8 16:35 00000001000000000000001C
-rw-------. 1 postgres postgres  16M Oct  8 16:35 00000001000000000000001D
-rw-------. 1 postgres postgres  16M Oct  8 16:35 00000001000000000000001E
-rw-------. 1 postgres postgres  16M Oct  8 16:35 00000001000000000000001F
-rw-------. 1 postgres postgres  16M Oct  8 16:35 000000010000000000000020
-rw-------. 1 postgres postgres  16M Oct  8 16:35 000000010000000000000021
-rw-------. 1 postgres postgres  16M Oct  8 16:35 000000010000000000000022
-rw-------. 1 postgres postgres  16M Oct  8 16:35 000000010000000000000023
-rw-------. 1 postgres postgres  16M Oct  8 16:35 000000010000000000000024
-rw-------. 1 postgres postgres  16M Oct  8 16:35 000000010000000000000025
-rw-------. 1 postgres postgres  16M Oct  8 16:35 000000010000000000000026
-rw-------. 1 postgres postgres  16M Oct  8 16:35 000000010000000000000027
-rw-------. 1 postgres postgres  16M Oct  8 16:35 000000010000000000000028
-rw-------. 1 postgres postgres  16M Oct  8 16:35 000000010000000000000029
-rw-------. 1 postgres postgres  16M Oct  8 16:35 00000001000000000000002A
-rw-------. 1 postgres postgres  16M Oct  8 16:35 00000001000000000000002B
-rw-------. 1 postgres postgres  16M Oct  8 16:35 00000001000000000000002C
-rw-------. 1 postgres postgres  16M Oct  8 16:35 00000001000000000000002D
-rw-------. 1 postgres postgres  16M Oct  8 16:35 00000001000000000000002E
-rw-------. 1 postgres postgres  16M Oct  8 16:35 00000001000000000000002F
-rw-------. 1 postgres postgres  16M Oct  8 16:35 000000010000000000000030
-rw-------. 1 postgres postgres  16M Oct  8 16:35 000000010000000000000031
-rw-------. 1 postgres postgres  16M Oct  8 16:35 000000010000000000000032
-rw-------. 1 postgres postgres  16M Oct  8 16:35 000000010000000000000033
-rw-------. 1 postgres postgres  16M Oct  8 16:35 000000010000000000000034
-rw-------. 1 postgres postgres  16M Oct  8 16:36 000000010000000000000035
drwx------. 2 postgres postgres 4.0K Oct  8 16:36 archive_status
-rw-------. 1 postgres postgres  16M Oct  8 16:36 xlogtemp.7289
[postgres@lxicbpgdsgv01 ~]$ 

-- Now We can see Many WAL Files (Redo log files) generated, count increased from 3 WAL files to 50+ WAL files

View Archived WAL Files (Archive log files)

postgres=# SHOW archive_command;
        archive_command
-------------------------------
 cp %p /pgArch/pgsql17/arch/%f
(1 row)

postgres=#

[postgres@lxicbpgdsgv01 ~]$ 
[postgres@lxicbpgdsgv01 ~]$ ls -lrth /pgArch/pgsql17/arch/
total 976M
-rw-------. 1 postgres postgres 16M Oct  7 23:50 000000010000000000000001
-rw-------. 1 postgres postgres 16M Oct  7 23:51 000000010000000000000002
-rw-------. 1 postgres postgres 16M Oct  8 01:28 000000010000000000000003
-rw-------. 1 postgres postgres 16M Oct  8 01:28 000000010000000000000004
-rw-------. 1 postgres postgres 16M Oct  8 04:24 000000010000000000000005
-rw-------. 1 postgres postgres 16M Oct  8 16:08 000000010000000000000006
-rw-------. 1 postgres postgres 16M Oct  8 16:24 000000010000000000000007
-rw-------. 1 postgres postgres 16M Oct  8 16:25 000000010000000000000008
-rw-------. 1 postgres postgres 16M Oct  8 16:34 000000010000000000000009
-rw-------. 1 postgres postgres 16M Oct  8 16:34 00000001000000000000000A
-rw-------. 1 postgres postgres 16M Oct  8 16:34 00000001000000000000000B
-rw-------. 1 postgres postgres 16M Oct  8 16:34 00000001000000000000000C
-rw-------. 1 postgres postgres 16M Oct  8 16:34 00000001000000000000000D
-rw-------. 1 postgres postgres 16M Oct  8 16:34 00000001000000000000000E
-rw-------. 1 postgres postgres 16M Oct  8 16:34 00000001000000000000000F
-rw-------. 1 postgres postgres 16M Oct  8 16:34 000000010000000000000010
-rw-------. 1 postgres postgres 16M Oct  8 16:35 000000010000000000000011
-rw-------. 1 postgres postgres 16M Oct  8 16:35 000000010000000000000012
-rw-------. 1 postgres postgres 16M Oct  8 16:35 000000010000000000000013
-rw-------. 1 postgres postgres 16M Oct  8 16:35 000000010000000000000014
-rw-------. 1 postgres postgres 16M Oct  8 16:35 000000010000000000000015
-rw-------. 1 postgres postgres 16M Oct  8 16:35 000000010000000000000016
-rw-------. 1 postgres postgres 16M Oct  8 16:35 000000010000000000000017
-rw-------. 1 postgres postgres 16M Oct  8 16:35 000000010000000000000018
-rw-------. 1 postgres postgres 16M Oct  8 16:35 000000010000000000000019
-rw-------. 1 postgres postgres 16M Oct  8 16:35 00000001000000000000001A
-rw-------. 1 postgres postgres 16M Oct  8 16:35 00000001000000000000001B
-rw-------. 1 postgres postgres 16M Oct  8 16:35 00000001000000000000001C
-rw-------. 1 postgres postgres 16M Oct  8 16:35 00000001000000000000001D
-rw-------. 1 postgres postgres 16M Oct  8 16:35 00000001000000000000001E
-rw-------. 1 postgres postgres 16M Oct  8 16:35 00000001000000000000001F
-rw-------. 1 postgres postgres 16M Oct  8 16:35 000000010000000000000020
-rw-------. 1 postgres postgres 16M Oct  8 16:35 000000010000000000000021
-rw-------. 1 postgres postgres 16M Oct  8 16:35 000000010000000000000022
-rw-------. 1 postgres postgres 16M Oct  8 16:35 000000010000000000000023
-rw-------. 1 postgres postgres 16M Oct  8 16:35 000000010000000000000024
-rw-------. 1 postgres postgres 16M Oct  8 16:35 000000010000000000000025
-rw-------. 1 postgres postgres 16M Oct  8 16:35 000000010000000000000026
-rw-------. 1 postgres postgres 16M Oct  8 16:35 000000010000000000000027
-rw-------. 1 postgres postgres 16M Oct  8 16:35 000000010000000000000028
-rw-------. 1 postgres postgres 16M Oct  8 16:35 000000010000000000000029
-rw-------. 1 postgres postgres 16M Oct  8 16:35 00000001000000000000002A
-rw-------. 1 postgres postgres 16M Oct  8 16:35 00000001000000000000002B
-rw-------. 1 postgres postgres 16M Oct  8 16:35 00000001000000000000002C
-rw-------. 1 postgres postgres 16M Oct  8 16:35 00000001000000000000002D
-rw-------. 1 postgres postgres 16M Oct  8 16:35 00000001000000000000002E
-rw-------. 1 postgres postgres 16M Oct  8 16:35 00000001000000000000002F
-rw-------. 1 postgres postgres 16M Oct  8 16:35 000000010000000000000030
-rw-------. 1 postgres postgres 16M Oct  8 16:35 000000010000000000000031
-rw-------. 1 postgres postgres 16M Oct  8 16:35 000000010000000000000032
-rw-------. 1 postgres postgres 16M Oct  8 16:35 000000010000000000000033
-rw-------. 1 postgres postgres 16M Oct  8 16:35 000000010000000000000034
-rw-------. 1 postgres postgres 16M Oct  8 16:36 000000010000000000000035
-rw-------. 1 postgres postgres 16M Oct  8 16:36 000000010000000000000036
-rw-------. 1 postgres postgres 16M Oct  8 16:36 000000010000000000000037
-rw-------. 1 postgres postgres 16M Oct  8 16:36 000000010000000000000038
-rw-------. 1 postgres postgres 16M Oct  8 16:36 000000010000000000000039
-rw-------. 1 postgres postgres 16M Oct  8 16:36 00000001000000000000003A
-rw-------. 1 postgres postgres 16M Oct  8 16:36 00000001000000000000003B
-rw-------. 1 postgres postgres 16M Oct  8 16:36 00000001000000000000003C
-rw-------. 1 postgres postgres 16M Oct  8 16:36 00000001000000000000003D
[postgres@lxicbpgdsgv01 ~]$

-- WAL and Archive File Size: When archived, the exact same 16MB WAL segment file is copied as raw binary to the archive destination. Therefore, archive files always match the WAL segment size (redo log size).

Best Practices

Use a dedicated mount point for pg_wal (WAL directory) – eg. /pgWal
Set min_wal_size to higher value in production (e.g. 8GB)
Keep WAL archive files on a separate mount point for better I/O and space isolation – eg. /pgArch
Monitor WAL archive directory regularly to avoid disk full errors

Note:

WAL file size is fixed at cluster init (default: 16MB). You cannot change it for an existing cluster.
PostgreSQL WAL Size Be Can Increased only at NEW cluster creation time via initdb (64MB)
/usr/pgsql-17/bin/initdb –wal-segsize=64 -D /pgData/pgsql17/data

Caution: Your use of any information or materials on this website is entirely at your own risk. It is provided for educational purposes only. It has been tested internally, however, we do not guarantee that it will work for you. Ensure that you run it in your test environment before using.

Thank you,
Rajasekhar Amudala
Email: br8dba@gmail.com
Linkedin: https://www.linkedin.com/in/rajasekhar-amudala/

Bright DBA

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WAL Files