If you don’t know the importance of pg_xlog, please visit a below article.

PostgreSQL: Difference between pg_log, pg_clog and pg_xlog log directories

Recently, we were facing a size related problem in one of our PostgreSQL test servers. A system admin found that the size of pg_xlog folder is large. What he did was, he deleted few log files from pg_xlog folder and restarted the PostgreSQL service without knowing the outcome.

Postgres server got down immediately and threw error like ‘PG_CTL failed’.

A pg_xlog folder contains the main transaction log files (WAL files) which are necessary to load the database and other objects. The size of pg_xlog folder is large, so if you are facing any size related problem, you can archive it.

The very first step to recovering your database using a backup file, but if you don’t have any backup of a database, you have to use pg_resetxlog utility which reset the pg_xlog.

pg_resetxlog may corrupt your database, or you may get data loss, as the documentation warns because of missing WAL files.
After running this command, it should be possible to start the server but bear in mind that the database might contain inconsistent data due to partially-committed transactions.

Step 1:

Go to /usr/bin/ directory

Step 2: Execute pg_resetxlog utility to reset pg_xlog

./pg_resetxlog -f /data/

In this post, I am sharing the basic purpose of pg_log, pg_clog and pg_xlog log directories in PostgreSQL.

If you look into your PostgreSQL data directory, you can find a different type of log folders.

pg_log:

This is a default database logs like: error messages, executed query log, dead lock information, Postgers start/stop messages.

First, you should look into this folder for any investigation. Most of the time we are getting an error because of bad queries or wrong configuration so in these cases we can find the important log messages from this folder.

You can also delete unwanted or old log files because It stores only database action related messages which are not critical for PostgreSQL Server.
There are a few parameters in Postgres.conf file related to pg_log folder, you access below article for this.

PostgreSQL: Important Parameters to enable Log for all Queries

pg_clog:

We cannot read log files of this folder because it is for PostgreSQL internals. It contains logs for transaction metadata. PostgreSQL server access this folder to take the decision like which transaction completed or which did not.

This is critical log folder, you should never forget this folder during your pg_data directory backup.

pg_xlog:

At all the times, PostgreSQL maintains a write-ahead log (WAL) in the pg_xlog/ subdirectory of the cluster’s data directory. The log records for every change made to the database’s data files. These log messages exists primarily for crash-safety purposes.

It contains the main binary transaction log data or binary log files. If you are planning for replication or Point in time Recovery, we can use this transaction log files.

We cannot delete this file. Otherwise, it causes a database corruption. The size of this folder would be greater than actual data so If you are dealing with massive database, 99% chance to face disk space related issues especially for the pg_xlog folder.

But don’t worry, There are multiple ways to optimize pg_xlog folder by removing old WAL files.