3.8. Backing up and restoring PostgreSQL data with continuous archiving

Procedure

1. In the /var/lib/pgsql/data/postgresql.conf file:

   1. Set the wal_level configuration parameter to replica or higher.
   2. Set the archive_mode parameter to on.

   3. Specify the shell command in the archive_command configuration parameter. You can use the cp command, another command, or a shell script.

      For example:

      archive_command = 'test ! -f /mnt/server/archivedir/%f && cp %p /mnt/server/archivedir/%f'

      where the %p parameter is replaced by the relative path to the file to archive and the %f parameter is replaced by the file name.

      This command copies archivable WAL segments to the /mnt/server/archivedir/ directory. After replacing the %p and %f parameters, the executed command looks as follows:

      test ! -f /mnt/server/archivedir/00000001000000A900000065 && cp pg_wal/00000001000000A900000065 /mnt/server/archivedir/00000001000000A900000065

      A similar command is generated for each new file that is archived.

      注記

      The archive command is executed only on completed WAL segments. A server that generates little WAL traffic can have a substantial delay between the completion of a transaction and its safe recording in archive storage. To limit how old unarchived data can be, you can:

      • Set the archive_timeout parameter to force the server to switch to a new WAL segment file with a given frequency.
      • Use the pg_switch_wal parameter to force a segment switch to ensure that a transaction is archived immediately after it finishes.

2. Restart the postgresql service to enable the changes:

   # systemctl restart postgresql.service

3. Test your archive command and ensure it does not overwrite an existing file and that it returns a nonzero exit status if it fails.
4. To protect your data, ensure that the segment files are archived into a directory that does not have group or world read access.

Procedure

1. Use the pg_basebackup utility to perform the base backup.

   • To create a base backup as individual files (plain format):

     $ pg_basebackup -D <backup_directory> -Fp

     Replace backup_directory with your chosen backup location.

     If you use tablespaces and perform the base backup on the same host as the server, you must also use the --tablespace-mapping option, otherwise the backup will fail upon an attempt to write the backup to the same location.

   • To create a base backup as a tar archive (tar and compressed format):

     $ pg_basebackup -D <backup_directory> -Ft -z

     Replace backup_directory with your chosen backup location.

     To restore such data, you must manually extract the files in the correct locations.

   To specify which database server pg_basebackup will contact, use the following command-line options:

   • The -h option to define the host.

     The default host is either the local host or a host specified by the PGHOST environment variable.

   • The -p option to define the port.

     The default port is indicated by the PGPORT environment variable or the compiled-in default.

2. After the base backup process is complete, safely archive the copy of the database cluster and the WAL segment files used during the backup, which are specified in the backup history file.
3. Delete WAL segments numerically lower than the WAL segment files used in the base backup because these are older than the base backup and no longer needed for a restore.

Procedure

1. Stop the server:

   # systemctl stop postgresql.service

2. Copy the necessary data to a temporary location.

   Preferably, copy the whole cluster data directory and any tablespaces. Note that this requires enough free space on your system to hold two copies of your existing database.

   If you do not have enough space, save the contents of the cluster’s pg_wal directory, which can contain logs that were not archived before the system went down.

3. Remove all existing files and subdirectories under the cluster data directory and under the root directories of any tablespaces you are using.

4. Restore the database files from your base backup.

   Ensure that:

   • The files are restored with the correct ownership (the database system user, not root).
   • The files are restored with the correct permissions.
   • The symbolic links in the pg_tblspc/ subdirectory are restored correctly.

5. Remove any files present in the pg_wal/ subdirectory.

   These files resulted from the base backup and are therefore obsolete. If you did not archive pg_wal/, re-create it with proper permissions.

6. Copy any unarchived WAL segment files that you saved in step 2 into pg_wal/.

7. Create the recovery.conf recovery command file in the cluster data directory and specify the shell command in the restore_command configuration parameter. You can use the cp command, another command, or a shell script. For example:

   restore_command = 'cp /mnt/server/archivedir/%f "%p"'

8. Start the server:

   # systemctl start postgresql.service

   The server enters the recovery mode and proceeds to read through the archived WAL files that it needs.

   If the recovery is terminated due to an external error, the server can be restarted, and it will continue the recovery. When the recovery process is completed, the server renames recovery.conf to recovery.done. This prevents the server from accidentally re-entering the recovery mode after it starts normal database operations.

9. Check the contents of the database to verify that the database has recovered into the required state.

   If the database has not recovered into the required state, return to step 1. If the database has recovered into the required state, allow the users to connect by restoring the client authentication configuration in the pg_hba.conf file.