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PostgreSQL is a widely used relational database that supports ACID transactions. The acronym ACID stands for atomicity, consistency, isolation, and durability. These are four key properties of database transactions that PostgreSQL supports to ensure the persistence and validity of data in the database.
One method PostgreSQL uses to maintain ACID properties is Write-Ahead Logging (WAL). PostgreSQL first records any transaction on the database to the WAL log files before it writes the changes to the database cluster’s data files.
With continuous archiving, the WAL files are copied to secondary storage, which has a couple of benefits. For example, a secondary database cluster can use the archived WAL file for replication purposes, but you can also use the files to perform point-in-time-recovery (PITR). That is, you can use the files to rollback a database cluster to a desirable point if an accident happens.
In this tutorial, you will set up continuous archiving with a PostgreSQL 12 cluster on Ubuntu 20.04 and perform PITR on the cluster.
To complete this tutorial, you’ll need the following:
A single machine running Ubuntu 20.04. The Initial Server Setup Guide will walk you through setting up the server including non-root users with the appropriate permissions.
PostgreSQL 12 running on the machine. Step 1 of the How To Install and Use PostgreSQL on Ubuntu 20.04 tutorial covers the installation of PostgreSQL 12 on Ubuntu 20.04.
In this first step, you need to configure your PostgreSQL 12 cluster to archive the cluster’s WAL files in a directory different from the cluster’s data directory. To do this, you must first create a new directory somewhere to archive the WAL files.
Create a new directory as follows:
- mkdir database_archive
You now need to give the default PostgreSQL user,
postgres, permission to write to this directory. You can achieve this by changing the ownership of the directory using the
- sudo chown postgres:postgres database_archive
Now that you have a directory set up for the cluster to archive the WAL files into, you must enable archiving in the
postgresql.conf configuration file, which you can find in the
/etc/postgresql/12/main/ directory by default.
Open the configuration file with your text editor:
- sudo nano /etc/postgresql/12/main/postgresql.conf
Once you have opened the file, you’ll uncomment the line with the
archive_mode variable on it by removing the
# from the start of the line. Also, change the value of
on like the following:
. . . archive_mode = on . . .
You’ll also specify the command the cluster uses to archive the files. PostgreSQL provides an archive command that will work for this tutorial, which you can read about in the official PostgreSQL docs. Uncomment the
archive_command variable and add the following command:
. . . archive_command = 'test ! -f /path/to/database_archive/%f && cp %p /path/to/database_archive/%f' . . .
The archive command here first checks to see if the WAL file already exists in the archive, and if it doesn’t, it copies the WAL file to the archive.
/path/to/database_archive with the path to the
database_archive directory you created earlier. For example, if you created this in your home directory:
Lastly, you need to configure the
wal_level dictates how much information PostgreSQL writes to the log. For continuous archiving, this needs to be set to at least
. . . #wal_level = replica . . .
This is already the default value in PostgreSQL 12, so you shouldn’t need to change it, but it is something to remember if you ever go to change this variable.
You can now save and exit your file.
To implement the changes to your database cluster configuration file, you need to restart the cluster as follows:
- sudo systemctl restart postgresql@12-main
If PostgreSQL restarts successfully, the cluster will archive every WAL file once it is full. By default, each WAL file is 16MB.
In the case that you need to archive a transaction immediately, you can force the database cluster to change and archive the current WAL file by running the following command on the cluster:
- sudo -u postgres psql -c "SELECT pg_switch_wal();"
With the database cluster successfully copying the WAL files to the archive, you can now perform a physical backup of the database cluster’s data files.
It is important to take regular backups of your database to help mitigate data loss should the worst happen. PostgreSQL allows you to take both logical and physical backups of the database cluster. However, for PITR, you need to take a physical backup of the database cluster. That is, you need to make a copy of all the database’s files in PostgreSQL’s data directory. By default, the PostgreSQL 12 data directory is
Note: You can also find the location of the data directory by running the following command on the cluster:
- sudo -u postgres psql -c "SHOW data_directory;"
In the previous step, you made the directory,
database_archive, to store all the archived WAL files. In this step you need to create another directory, called
database_backup, to store the physical backup you will take.
Once again, make the directory:
- mkdir database_backup
Now ensure that the
postgres user has permission to write to the directory by changing the ownership:
- sudo chown postgres:postgres database_backup
Now that you have a directory for the backup, you need to perform a physical backup of the database cluster’s data files. Fortunately, PostgreSQL has the built-in
pg_basebackup command that performs everything for you. Run the command as the
- sudo -u postgres pg_basebackup -D /path/to/database_backup
/path/to/ with the path to your directory.
With this physical backup of the database cluster, you are now able to perform point-in-time-recovery on the cluster.
Now that you have at least one physical backup of the database and you’re archiving the WAL files, you can now perform PITR, if you need to rollback the database to a previous state.
First, if the database is still running, you’ll need to shut it down. You can do this by running the
systemctl stop command:
- sudo systemctl stop postgresql@12-main
Once the database is no longer running, you need to remove all the files in PostgreSQL’s data directory. But first, you need to move the
pg_wal directory to a different place as this might contain unarchived WAL files that are important for recovery. Use the
mv command to move the
pg_wal directory as follows:
- sudo mv /var/lib/postgresql/12/main/pg_wal ~/
Now, you can remove the
/var/lib/postgresql/12/main directory entirely and recreate it as such:
- sudo rm -rf /var/lib/postgresql/12/main
- sudo mkdir /var/lib/postgresql/12/main
Now, you need to copy all the files from the physical backup you made in the previous step to the new empty data directory. You can do this with
- sudo cp -a /path/to/database_backup/. /var/lib/postgresql/12/main/
You also need to ensure the data directory has the
postgres user as the owner and the appropriate permissions. Run the following command to change the owner:
- sudo chown postgres:postgres /var/lib/postgresql/12/main
And update the permissions:
- sudo chmod 700 /var/lib/postgresql/12/main
The WAL files in the
pg_wal directory copied from the physical backup are outdated and not useful. You need to replace them with the WAL files in the
pg_wal directory that you copied before you emptied out the PostgreSQL’s data directory as some of the files might not have been archived before stopping the server.
pg_wal file in the
/var/lib/postgresql/12/main directory as follows:
- sudo rm -rf /var/lib/postgresql/12/main/pg_wal
Now copy the files from the
pg_wal directory you saved before clearing out the data directory:
- sudo cp -a ~/pg_wal /var/lib/postgresql/12/main/pg_wal
With the data directory restored correctly, you need to configure the recovery settings to ensure the database server recovers the archived WAL files correctly. The recovery settings are found in the
postgresql.conf configuration file in the
Open the configuration file:
- sudo nano /etc/postgresql/12/main/postgresql.conf
Once you have the file open, locate the
restore_command variable and remove the
# character from the start of the line. Just like you did with
archive_command in the first step, you need to specify how PostgreSQL should recover the WAL files. Since the
archive command just copies the files to the archive, the
restore command will copy the files back. The
restore_command variable will be similar to the following:
. . . restore_command = 'cp /path/to/database_archive/%f %p' . . .
Remember to replace
/path/to/database_archive/ with the path to your archive directory.
Next, you have the option to specify a recovery target. This is the point that the database cluster will try to recover to before leaving recovery mode. The recovery target can be a timestamp, transaction ID, log sequence number, the name of a restore point created with the
pg_create_restore_point() command, or whenever the database reaches a consistent state. If no recovery target is specified, the database cluster will read through the entire log of WAL files in the archive.
For a complete list of options for the
recovery_target variable, consult the official PostgreSQL documentation.
Note: The recovery target must be a point in time after the physical backup you are using was taken. If you need to return to an earlier point, then you need to use an earlier backup of the database.
Once you have set
recovery_target, save and exit the file.
Before restarting the database cluster, you need to inform PostgreSQL that it should start in recovery mode. You can achieve this by creating an empty file in the cluster’s data directory called
recovery.signal. To create an empty file in the directory, use the
- sudo touch /var/lib/postgresql/12/main/recovery.signal
Now you can restart the database cluster by running:
- sudo systemctl start postgresql@12-main
If the database started successfully, it will enter recovery mode. Once the database cluster reaches the recovery target, it will remove the
Now that you have successfully recovered your database cluster to the desired state, you can begin your normal database operations. If you want to recover to a different point in time, you can repeat this step.
In this tutorial, you set up a PostgreSQL 12 database cluster to archive WAL files and then you used the archived WAL files to perform point-in-time-recovery. You can now rollback a database cluster to a desirable state if an accident happens.
To learn more about continuous archiving and point-in-time-recovery, you can read the docs.
For more tutorials on PostgreSQL, check out our PostgreSQL topic page.
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