In this post we cover the methods used to achieve an enterprise-grade backup strategy for the PostgreSQL cluster. In setting up our demonstration system, we use pg_basebackup and continuous archiving. The size of the database and your database environment—if it is on colo or cloud—matters. It is very important to understand how we can ensure minimalistic or no data loss at the time of disasters using our preferred backup tools.
As discussed in the introductory blog post, in our webinar of October 10, 2018 we highlight important aspects an enterprise should consider for their PostgreSQL environments. In our last blog post we looked at security.
The ability to recover a database to a certain point in time is always the ultimate aim of a backup strategy. Without a recoverable backup, you have no backup! So there are always two parts to a backup strategy: backup AND recovery. The backup tool you use to achieve your aims may vary. Let’s discuss some of the options.
The community version of PostgreSQL comes with a trusted backup tool called pg_basebackup. This utility can help you take a consistent online binary backup that is very fast, can be used for point-in-time-recovery, and also for setting up slaves/replicas.
Archiving and continuous archiving
Postgres does not flush data pages to disk upon every transaction commit. To ensure that PostgreSQL achieves durability and endures crash recovery, changes are written to transactions logs (a.k.a. WALs, Write-Ahead Logs) stored on disk.
WALs in PostgreSQL are similar to transaction log files in the InnoDB storage engine for MySQL. However, WAL files are recycled according to the values set to the parameters wal_keep_segments and max_wal_size. Hence, if WAL files are not copied to a safe location—such as a backup server or another file system—it won’t be possible to achieve point-in-time-recovery (PITR).
In order to archive WAL segments to a safe location, the parameter archive_mode must be set to ‘ON’ and we must pass an appropriate shell command to the archive_command parameter. The shell command can be a copy (cp command in Linux) or a bash script that tells postgres where to archive its WAL files. Administrators commonly use an NFS share or SAN partition for this, mounted locally to facilitate the storage of these files.You may refer to the manual page PostgreSQL Archiving to learn more about archiving.
For our demonstration set up, and taking into account the size of the database and the number of transactions, we use the following backup strategy:
- Nightly pg_basebackup that runs every day at 01:00 am.
- Continuous archiving of WAL’s to a remote backup server that is redundant and avoids single point of failure.
Using the above strategy, we can ensure that we are storing full backups daily. At the same time, the changes/transactions are backed up using the archiving solution available with PostgreSQL. In this way, we can achieve recovery to any point-back-in-time, depending on our backup retention policies.
Be sure to check out our webinar, Enterprise-grade PostgreSQL Built on Open Source Tools, to see this in practice. If you are reading this in time to join us on October 10th, you’ll have the chance to ask questions, too.
Depending on your database size, number of transactions and the environment, you may also consider using pgBackRest, Barman or WAL-g. These are stable open source backup solutions that have been contributed and are continuously maintained by community members. They help us to achieve one or more of the following features:
- Incremental backups
- Differential backups
- Features for building standby replicas
- Ability to stream backups to another server
- Streaming backups to AWS S3 or object store, removing the need to store backups locally or on locally mounted network shares before uploading to the cloud.
Here is a snippet of what is/isn’t possible with each of these backup tools today.