I really love PostgreSQL.
It has a really good performance, has a large eco-system, has a maltitude of extensions for every special case and - what I love the most is rock-solid and offers an almost complete SQL standard implementation.
When you start using (and developing with) it, you most likely use
pg_dump or pg_dumpall for backups.
Once started they create a single SQL file containing both DDL and DML statements to restore or copy your database or a complete cluster (PostgreSQL terminology:the sum of all databases under control of a single PostgreSQL service, e.g. all databases on one machine).
Sooner or later you go into production and you want to minimize downtime - and thus dataloss - to the absolute minimum possible.
With pg_dump you can only call it more often (from once a day to every hour) but sooner or later your database size will prevent the backup to be taken before the next run is started.
One escape from this is to enable point-in-time-recovery.
After a default installation PostgreSQL is run in a non-archive mode.
That means that all changes are written to the WAL, Write Ahead Log, before they finally make it to the database files.
This ensures entegrity in the event of an outage or filesystem error.When the root cause is cleared and you restart the PostgreSQL server it reads the WAL and applies all changes or discards unfinished transactions.
So the first step to enable PITR is to enable WAL-archiving. That basically means the finished WAL files are copied to a archive destination.
To get an idea please assume to have an archive directory structure as this,to which the postgres user has full access:
mkdir /archive/base mkdir /archive/wal chown postgres:postgres /archive/base /archive/wal /archive/ |-- base |-- wal
Enable the following settings in postgresql.conf:
wal_level = replica archive_mode = on archive_command = 'cp %p /archive/wal/%f' max_wal_senders = 1
Add the following line to pg_hba.conf:
local replication postgres peer
After doing so, PostgreSQL will copy all finished WAL files to the archive directory.
Now you can take a full database backup any time by issuing commands like these:
mkdir /archive/base/$(date +%Y-%m-%d) pg_basebackup -D /archive/base/$(date +%Y-%m-%d)
Once pg_basebackup completes you have a complete backup of the PostgreSQL cluster data:
tree /archive/base/ -L 2 /archive/base/`-- 2019-03-02 |-- backup_label |-- base |-- global |-- pg_clog |-- pg_commit_ts |-- pg_dynshmem |-- pg_logical |-- pg_multixact |-- pg_notify |-- pg_replslot |-- pg_serial |-- pg_snapshots |-- pg_stat |-- pg_stat_tmp |-- pg_subtrans |-- pg_tblspc |-- pg_twophase |-- PG_VERSION |-- pg_xlog `-- postgresql.auto.conf18 directories, 3 files
Also, you can see that there are WAL archive files, with one of it marked to be a special file for restores:
tree /archive/wal/ -L 2 /archive/wal/ |-- 000000010000000000000001 |-- 000000010000000000000002 |-- 000000010000000000000003 |-- 000000010000000000000004 |-- 000000010000000000000005 |-- 000000010000000000000006` -- 000000010000000000000006.00000028.backup
The one which ends on .backup contains information about when the backup was started and when it ended, so that a restore does not need to read and apply all WAL archive files in the archive, but only those written during and after it.
Now, lets create some random data to see that we have successfully enabled PITR and are able to use it.
CREATE DATABASE SAMPLE4 TEMPLATE template0; \C SAMPLE4; -- connect to database SAMPLE4 CREATE TABLE INFO (ID INTEGER, TITLE VARCHAR(20)); INSERT INTO INFO (ID,TITLE) VALUES (1,'23:10'); SELECT pg_switch_wal(); -- force switching WAL
After that you should see a new WAL in the archive directory:
tree /archive/wal/ -L 2 /archive/wal/ |-- 000000010000000000000001 |-- 000000010000000000000002 |-- 000000010000000000000003 |-- 000000010000000000000004 |-- 000000010000000000000005 |-- 000000010000000000000006 |-- 000000010000000000000006.00000028.backup` -- 000000010000000000000007
P.S.: You should not use pg_switch_wal without a reason. Consult the PostgreSQL manual for information about WAL configuration and switch timings!
Next, we gonna kill the database:
[postgres@localhost data]$ pg_ctl -D /usr/local/pgsql/data/ stop waiting for server to shut down.... done server stopped [postgres@localhost data]$
At this point the database cluster is gone and you will see error messages,
when you try to start it.
So, time to recover!
Copy the base backup back to the clusters data directory:
cp-ar /archive/base/2019-03-02/* /usr/local/pgsql/data/
Create a file /usr/local/pgsql/recovery.conf with this content:
restore_command = 'cp /archive/wal/%f "%p"' recovery_target_timeline = 'latest'
P.S.: The recovery.conf file will be renamed to recovery.done by the PostgreSQL process once the restore is finished.
Then start the server and once the recovery process is done query some data:
\C SAMPLE4; select * from info; SAMPLE4 id | title ----+------- 1 | 23:10 1 | 23:11(2 rows)
If you take a look at the clusters log file you will notice, that it automatically recovered the WAL archives and the most recent data is present.
This was a very simple setup; trust me, productive environments usually have more complexity (archiving WAL files to remote servers, restore from tape, make full backups by taking filesystem snapshots etc. etc.).
To make this example complete: add a cronjob entry which creates a full-backup every day, twice a day or even every hour and you should be able to recover in no time.