Updated May 17, 2023
Definition of PostgreSQL Wal
PostgreSQL provides a facility to the user to take backup, continue backup, and timely recovery from backup. This is the PostgreSQL tool’s main task, but how can we perform this operation. The answer to this question is that we use PostgreSQL WAL, and it minimizes disk I/O while we save the data. The WAL means Write Ahead Log today’s all database system uses WAL to provide durable and atomic transactions. The WAL stores the transaction in a sequential manner into the log file when we perform a write operation on the database. For the purpose of preserving database changes made on the server during a certain time period, the WAL is useful. However, after a certain period, the WAL is automatically discarded. The main advantages of WAL are it works as a replication of databases.
Syntax:
wal_keep_segmentExplanation:
In the above syntax, we use wal_keep_segment to specify the minimum number of previous log file segment stores into the pg_wal directory.
max_wal_sizeExplanation:
In the above syntax, we used max size with wal to specify the maximum size of the log file to grow the wal limit.
How Wal works in PostgreSQL?
Let’s see how WAL works in PostgreSQL as follows.
When we make some changes in our database, the WAL file maintains changes of your database into the pg_wal directory. The pg_wal directory is used in PostgreSQL version 10, and in the previous version, this directory we called pg_xlog. We can control all WAL files with the help of wal_keep_segments and the size of the WAL file by using max_wal_size. When we write data, they are added at the end of the WAL log file, and the log sequence number determines their position. The Log Sequence Number is uniquely identified into the transaction. Log Sequence Number is used to represent a position in WAL, that means when data is inserted or when written this kind of information, we can see with the help of Log Sequence Number. pg_lsn is a data type used to return Log Sequence Number.
How we can Configure Wal?
WAL configuration in PostgreSQL decided how databases work, for configuration WAL, we required two important systems, such as checkpoint and background writer. Let’s see the different memory areas used in WAL as follows.
1. Shared Buffers
The PostgreSQL.conf file contains a parameter known as “shared_buffers.” These buffers are referred to as “shared” because they are accessed by all background servers. The shared buffer is responsible for allocating memory to cache data, providing several key advantages such as enhanced system speed and rapid access to the data.
2. Dirty Pages
When updating or modifying data in memory, the changes are stored in a dirty page. It is crucial to understand that a dirty page and a shared buffer are different entities. Once the modified data is written to disk, it is then considered a dirty page.
3. Checkpointer
Checkpoints play a crucial role in the Write Ahead Log (WAL) system. They are utilized to create checkpoints within the WAL file. These checkpoints serve as reference points for recovery systems in the event of a system crash or transaction failure. The checkpoint’s main task is to show that all data has been updated at every checkpoint. The parameter “checkpoint_segments” indicates the maximum number of log segments that can exist between two checkpoints. On the other hand, “checkpoint_timeout” specifies the maximum time duration between two checkpoints. The default time interval between checkpoints is 5 minutes or 300 seconds.
4. Background Writer
The background writer, being a subsystem, plays a crucial role in the checkpoint process. It assists in alleviating the checkpoint’s workload by offloading some of the modified data to the disk.
Examples
Let us discuss examples of PostgreSQL Wal.
Example #1
Suppose the user needs to know the path of the pg_wal log directory at that time, you can use the following statement.
show data_directory;Explanation:
With the help of the above statement, we can see the pg_wal directory path. Illustrate the remaining end result of the above announcement by way of the usage of the following snapshot.
Let’s see the different examples to understand the working of WAL log files into PostgreSQL as follows.
Example #2
Suppose we need to find out the Wal keeps a segment of a past log file, so at that time, we use the following statement as follows.
cat postgresql.conf|grep -i wal_keep_segment;Explanation:
In the above statement, we use the cat command to find PostgreSQL.conf file, after that, we use the grep command to find out a specified segment from the PostgreSQL.conf file, that means we use the grep command with the wal_keep segment to know past log file segments. Illustrate the remaining end result of the above announcement by way of the usage of the following snapshot.
Example #3
Suppose users need to find out the maximum size of the wal file, so at that time, we can use the following syntax as follows.
cat postgresql.conf|grep -i max_wal_size;Explanation:
In the above statement, we use the cat command to find out postgresql.conf file from your system location after that, we use the grep command to find specific value from postgresql.conf file means that in the above statement, we use the grep command with max_wal_size to know the maximum size of the log file to grow the performance of the WAL log file. Illustrate the remaining end result of the above announcement by way of the usage of the following snapshot.
WAL Archiving
The main purpose of WAL in PostgreSQL is to secure data integrity. Mainly there are three approaches to backup data in PostgreSQL as follows.
1. Dump
We utilize the pg_dump utility in PostgreSQL to export the database and store its contents into a file. Primarily, this file serves as a means to back up the data, and the dump files maintain consistency by accurately representing the state of the database.
2. File Level Backup
This is the second approach to backup the data, and it is faster as compared with the first approach. In this approach, we directly copy files from the system, store them on another location, and use them as required.
3. Archiving
This point we already discussed in WAL, the main purpose of archiving in PostgreSQL is that we recover data when a system crash or any failure may occur at that time, we use checkpoint to recover data. To enable backup through replication, the WAL archiving must be configured. To activate the archiving mode, you can execute a specific command.
Conclusion- PostgreSQL Wal
We hope from this article, you have understood about the PostgreSQL WAL. From the above article, we have learned the basic syntax of PostgreSQL WAL. We have additionally discovered how we can enforce them in PostgreSQL with different examples of every technique. From this article, we have learned how we can handle WAL in PostgreSQL.
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