Definition of PostgreSQL Wal
PostgreSQL provides facility to the user to take backup, continue backup and timely recovery from backup. This is the main task of PostgreSQL tool but how we can 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. The WAL is useful to maintain modification of the database on the server for a particular time period after that it is automatically discarded. The main advantages of WAL are, it works as a replication of databases.
Syntax:
wal_keep_segment
Explanation:
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_size
Explanation:
In the above syntax, we used max size with wal to specify maximum size of log file to grow 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 file with the help of wal_keep_segments and we also control 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 its position is determined by log sequence number. 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 different memory areas used in WAL as follows.
1. Shared Buffers
In the PostgreSQL.conf file, there is one parameter we called shared buffers. They are called shared buffers because all background servers access the shared buffers. The shared buffer is used to determine the amount memory is allocated for caching data that means the main advantages of a shared buffer are increased speed of the system and fast access.
2. Dirty Pages
When we update or modify data into memory that data is stored into a dirty page. The dirty page is different from a shared buffer and the disk is called a dirty page.
3. Checkpointer
The checkpoint is used to create a checkpoint into the Write Ahead Log file. These checkpoints are used in recovery systems when system crash or transaction failure. The main task of the checkpoint is to show all data has been updated at every checkpoint. The checkpoint_segment is used to show maximum log between two checkpoints and checkpoint_timout is used to show maximum time between two checkpoints, the default time between two checkpoints is 5 min or 300 sec.
4. Background Writer
Background writer is a subsystem and it is held at the checkpoint, it divides some load of checkpoint when modified data to the disk offloading the checkpoint.
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.
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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 working of WAL log files into PostgreSQL as follows.
Example #2
Suppose we need to find out the Wal keep 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 above statement, we use cat command to find PostgreSQL.conf file, after that we use grep command to find out a specified segment from PostgreSQL.conf file that means we use grep command with wal_keep segment to know past log file segment. . 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 above statement we use cat command to find out postgresql.conf file from your system location after that we use grep command to find specific value from postgresql.conf file that means in above statement we use grep command with max_wal_size to know the maximum size of log file to grow the performance of 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 to secure data integrity. Mainly there are three approaches to backup data in PostgreSQL as follows.
1. Dump
pg_dump utility is used to export the database in PostgreSQL and dump the content into the file. Mainly this file is used to backup the data and dump files are consistent that means they represent 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 and store on another location, and use them as per requirement.
3. Archiving
This point we already discussed in WAL, the main purpose of archiving in PostgreSQL is that we recover data when system crash or any failure may occur at that time we use checkpoint to recover data. The WAL archiving it must be set as a replica for backup and we activate archiving mode by using specified command.
Conclusion
We hope from this article you have understood about the PostgreSQL WAL. From the above article, we have learned the basic syntax 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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