Point in Time Recovery (PITR) using pg_basebackup in PostgreSQL

July 6, 2021

Point in Time Recovery (PITR) using pg_basebackup in PostgreSQL

In previous blog we saw How to perform Full recovery using backup taken from pg_basebackup Click here to read more.

In this blog we will see How to perform PITR using backup taken from pg_basebackup in PostgreSQL.

For the purpose of demonstration we will be using our second PostgreSql Cluster created on Port: 5433 Click here to learn How to create second PostgreSql Cluster. For pg_basebackup backup commands Click here to read more.

PostgreSQL Cluster Physical Structure: PostgreSQL Cluster data directory: /u01/psql-13, Created two non-default tablespaces myts03: /u01/psql3_tbls & myts04: /u01/psql4_tbls. Created two sample databases sample3 & sample4 and loaded them with sample data along with configured WAL archiving: /u01/wal_archive.

postgres=# show data_directory;
 data_directory
----------------
 /u01/psql-13
 
postgres=#  \db+
                                      List of tablespaces
    Name    |  Owner   |    Location     | Access privileges | Options |  Size   | Description
------------+----------+-----------------+-------------------+---------+---------+-------------
 myts03     | postgres | /u01/psql3_tbls |                   |         | 0 bytes |
 myts04     | postgres | /u01/psql4_tbls |                   |         | 0 bytes |
 pg_default | postgres |                 |                   |         | 46 MB   |
 pg_global  | postgres |                 |                   |         | 559 kB  |
(4 rows)
 
 
postgres=# \l+
                                                                    List of databases
   Name    |  Owner   | Encoding |   Collate   |    Ctype    |   Access privileges   |  Size   | Tablespace |                Description
-----------+----------+----------+-------------+-------------+-----------------------+---------+------------+--------------------------------------------
 dvdrental | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 | =Tc/postgres         +| 15 MB   | pg_default |
           |          |          |             |             | postgres=CTc/postgres+|         |            |
           |          |          |             |             | read_only=c/postgres  |         |            |
 postgres  | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 |                       | 15 MB   | pg_default | default administrative connection database
 sample3   | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 |                       | 15 MB   | myts03     |
 sample4   | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 |                       | 15 MB   | myts04     |
 template0 | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 | =c/postgres          +| 7969 kB | pg_default | unmodifiable empty database
           |          |          |             |             | postgres=CTc/postgres |         |            |
 template1 | postgres | UTF8     | en_US.UTF-8 | en_US.UTF-8 | =c/postgres          +| 8117 kB | pg_default | default template for new databases
           |          |          |             |             | postgres=CTc/postgres |         |            |
(6 rows)
 
postgres=#
 
postgres=# select name,setting,unit from pg_settings where name in ('archive_mode','archive_command','archive_timeout');
      name       |                          setting                           | unit
-----------------+------------------------------------------------------------+------
 archive_command | test ! -f /u01/wal_archive/%f && cp %p /u01/wal_archive/%f |
 archive_mode    | on

Create and Load test table and Switch WAL File: Let’s Create Table t_test and insert 1000 random records. Once insert operation is done do force WAL switch and make sure all changes are archived.

postgres=# \connect sample3
You are now connected to database "sample3" as user "postgres".
sample3=#
sample3=# create table t_Test (slno integer, ts timestamp);
CREATE TABLE
sample3=# insert into t_test values (generate_series(1,1000),now());
INSERT 0 1000
sample3=#
sample3=# select count(*) from t_test;
 count
-------
  1000
(1 row)
 
sample3=# \connect sample4
You are now connected to database "sample4" as user "postgres".
sample4=#
sample4=# create table t_Test (slno integer, ts timestamp);
CREATE TABLE
sample4=# insert into t_test values (generate_series(1,1000),now());
INSERT 0 1000
sample4=# select count(*) from t_test;
 count
-------
  1000
(1 row)
 
postgres=# select pg_switch_wal();
 pg_switch_wal
---------------
 0/A04DDC8
(1 row)
 
postgres=#
postgres=#

Full DB backup: Perform full cluster level tar backup using pg_basebackup using the below command.

-bash-4.2$ pg_basebackup -h localhost -p 5433 -U postgres -D /u01/backup/latest1_backup -Ft -z -Xs -P
78382/78382 kB (100%), 3/3 tablespaces
-bash-4.2$
 
-bash-4.2$ ls -ltr /u01/backup/latest1_backup
total 12900
-rw-------. 1 postgres postgres 2699207 Jul  4 16:16 21913.tar.gz
-rw-------. 1 postgres postgres 2699006 Jul  4 16:16 21914.tar.gz
-rw-------. 1 postgres postgres 7453080 Jul  4 16:16 base.tar.gz
-rw-------. 1 postgres postgres   17655 Jul  4 16:16 pg_wal.tar.gz
-rw-------. 1 postgres postgres  334189 Jul  4 16:16 backup_manifest
-bash-4.2$

Delete All Records: Let simulate human error that caused accidental delete of table data after 2021-07-05 11:04:28

sample4=#
sample4=# select now();
              now
-------------------------------
 2021-07-05 11:04:28.721948+03
(1 row)

sample4=#
sample4=# delete from t_test;
DELETE 1000
sample4=#
sample4=# \connect sample3
You are now connected to database "sample3" as user "postgres".
sample3=#
sample3=# delete from t_test;
DELETE 1000
sample3=#
sample3=# select now();
              now
-------------------------------
 2021-07-05 11:07:36.401521+03
(1 row)

sample3=#

Stop PostgreSql Cluster and remove all datafiles: In order to restore the complete postgresql cluster, we need to delete all datafiles in the base and tablespace directory.

[root@test-machine02 ~]# systemctl stop psql-13.service
[root@test-machine02 ~]# rm -rf /u01/psql-13/*
[root@test-machine02 ~]# rm -rf /u01/psql3_tbls/*
[root@test-machine02 ~]# rm -rf /u01/psql4_tbls/*

Follow the below steps to perform PITR

Step 1. Untar Base backup: Use OS command tar to copy the base backup to the base location: /u01/psql-13.

-bash-4.2$ tar xvf /u01/backup/latest1_backup/base.tar.gz -C  /u01/psql-13
backup_label
tablespace_map
pg_wal/
./pg_wal/archive_status/
global/
global/1262

Step 2. View tablespace_map File: pg_basebackup command creates a text file called tablespace_map which has mapping to tablespace OID and directory location.

-bash-4.2$ pwd
/u01/psql-13
-bash-4.2$ cat tablespace_map
21913 /u01/psql3_tbls
21914 /u01/psql4_tbls
-bash-4.2$

Step 3. Untar Tablespace & WAL Backup: Untar both tablespace backup and WAL backup to their respective location.

-bash-4.2$
-bash-4.2$ tar xvf /u01/backup/latest1_backup/21913.tar.gz -C  /u01/psql3_tbls
PG_13_202007201/
PG_13_202007201/21916/
PG_13_202007201/21916/1255
 
-bash-4.2$
-bash-4.2$ tar xvf /u01/backup/latest1_backup/21914.tar.gz -C  /u01/psql4_tbls
PG_13_202007201/
PG_13_202007201/21917/
PG_13_202007201/21917/1255
 
-bash-4.2$
-bash-4.2$ tar xvf /u01/backup/latest1_backup/pg_wal.tar.gz -C /u01/psql-13/pg_wal
00000001000000000000000C
archive_status/00000001000000000000000C.done
-bash-4.2$
-bash-4.2$

Step 4 : Update restore_command & recovery_target_time in postgresql.conf: Edit postgresql.conf file and provide WAL archiving location in restore_command and until the time we need to recover in recovery_target_time parameters. So postgresql service and do recovery based on provided target time.

-bash-4.2$ cd /u01/psql-13/
-bash-4.2$ vi postgresql.conf
restore_command = 'cp /u01/wal_archive/%f %p'
recovery_target_time = '2021-07-05 11:04:00 AST'
:wq!

-bash-4.2$ egrep -w 'restore_command|recovery_target_time' postgresql.conf
restore_command = 'cp /u01/wal_archive/%f %p'           # command to use to restore an archived logfile segment
recovery_target_time = '2021-07-05 11:04:00 AST'        # the time stamp up to which recovery will proceed
-bash-4.2$

Step 5. Start PostgreSQL Service: Once all the above steps are done. Start postgresql service.

[root@test-machine02 u01]# systemctl start psql-13.service
Job for psql-13.service failed because the control process exited with error code. See "systemctl status psql-13.service" and "journalctl -xe" for details.
[root@test-machine02 u01]# journalctl -xe
-- Unit psql-13.service has begun starting up.
Jul 05 11:16:07 test-machine02 postmaster[111318]: 2021-07-05 08:16:07.894 GMT [111318] LOG:  invalid value for parameter "recovery_target_time": "2021-07-05 11:04:00 AST"
Jul 05 11:16:07 test-machine02 postmaster[111318]: 2021-07-05 11:16:07.895 +03 [111318] FATAL:  configuration file "/u01/psql-13/postgresql.conf" contains errors
Jul 05 11:16:07 test-machine02 systemd[1]: psql-13.service: main process exited, code=exited, status=1/FAILURE
Jul 05 11:16:07 test-machine02 systemd[1]: Failed to start PostgreSQL 13 database server.
-- Subject: Unit psql-13.service has failed
-- Defined-By: systemd
-- Support: http://lists.freedesktop.org/mailman/listinfo/systemd-devel
--

Step 6. Update recovery_target_time: To fix above error edit parameter recovery_target_time and replace AST (Arabic Standard Time) to Asia/Riyadh (Depends on server/DB time zone).

-bash-4.2$ cd /u01/psql-13/
-bash-4.2$ vi postgresql.conf
restore_command = 'cp /u01/wal_archive/%f %p'
recovery_target_time = '2021-07-05 11:04:00 Asia/Riyadh'
:wq!

-bash-4.2$ egrep -w 'restore_command|recovery_target_time' postgresql.conf
restore_command = 'cp /u01/wal_archive/%f %p'           # command to use to restore an archived logfile segment
recovery_target_time = '2021-07-05 11:04:00 Asia/Riyadh'        # the time stamp up to which recovery will proceed
-bash-4.2$

Step 7. Start PostgreSQL Service: Once all the above steps are done. Start postgresql service.

[root@test-machine02 u01]#
[root@test-machine02 u01]# systemctl start psql-13.service
[root@test-machine02 u01]#

Step 8. Verify logfile: Verify logfile to validate everything went fine while restoring.

-bash-4.2$ pwd
/u01/psql-13/log
bash-4.2$ vi postgresql-Mon.log

2021-07-05 11:24:21.430 +03 [111753] LOG:  listening on Unix socket "/var/run/postgresql/.s.PGSQL.5433"
2021-07-05 11:24:21.432 +03 [111753] LOG:  listening on Unix socket "/tmp/.s.PGSQL.5433"
2021-07-05 11:24:21.434 +03 [111757] LOG:  database system was interrupted; last known up at 2021-07-04 16:16:13 +03
2021-07-05 11:24:21.453 +03 [111757] LOG:  redo starts at 0/C000028
2021-07-05 11:24:21.453 +03 [111757] LOG:  consistent recovery state reached at 0/C000100
2021-07-05 11:24:21.453 +03 [111757] LOG:  redo done at 0/C000100
2021-07-05 11:24:21.532 +03 [111753] LOG:  database system is ready to accept connections

Step 9: Verify Data: Verify all data exists after restore and recovery.

-bash-4.2$ psql -p 5433
psql (13.2)
Type "help" for help.

postgres=#
postgres=# \connect sample3
You are now connected to database "sample3" as user "postgres".
sample3=# select count(*) from t_test;
 count
-------
  1000
(1 row)

sample3=# \connect sample4
You are now connected to database "sample4" as user "postgres".
sample4=# select count(*) from t_test;
 count
-------
  1000
(1 row)

sample4=#

This document is just for learning purpose and always validate in the LAB environment first before applying in the LIVE environment.

Hope so you like this article!
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Point in Time Recovery (PITR) using pg_basebackup in PostgreSQL

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