Backup types explained: Full, incremental, differential, synthetic
Backup is very much an evergreen fundamental of IT. Even if your organisation uses snapshots or replication to secondary storage or datacentres, backup is still vital.
That’s because backups provide a trusted copy of your data to which you can roll back, and are an IT fundamental that has seen renewed focus in the light of rising ransomware attacks.
Snapshots are not copies in the same way. Snapshots are built of numerous pointers to the state of data at a point in time, but which could be created from information assembled over a long period as parts of files, directories, etc changed. So backups are still needed.
Wherever you need backup – whether source and/or target are on-site or in the cloud – there are several types of backup you need to know about. These include full, differential, incremental, and hybrids of these, such as synthetic and incremental-forever. We will outline them here.
The emergence of the cloud as a site for compute and storage or as a target for backup retention doesn’t change much here, either. Operations that run purely in the cloud still often need actual backup, because default data protection may not fit many organisations’ requirements in terms of compliance or simple reliability.
The big cloud providers – AWS, Azure and Google Cloud – all offer backup services that are mostly a combination of full and incrementals, but we deal with that in other articles.
Where operations are hybrid cloud, the cloud often forms the target where backups are stored. But even here, it is likely – for reasons of access time to recently changed data – that backups are made available on local storage.
A full backup is where all data in a specific dataset is copied. It is usually done at the outset of deploying a backup solution and possibly at regular intervals – although that depends on customer choices. Because it encompasses an entire dataset, it is the most time-consuming and takes up the most storage capacity. Advantages are that a full backup may occupy less space than a lengthy set of changed-data backups and can be easier to restore from.
With a full backup already completed at deployment and then possibly repeated on a regular basis, incremental backups copy only data changed since the last backup. That makes incrementals the least time- and storage space-consuming method of backup. To restore data, you must rebuild it from the last full backup plus all subsequent incremental backups.
Differential backups make a copy of all data that has changed since the last full, so restores need the last full backup and just the latest differential. That makes restores potentially less complex than with a full-plus-incremental backup regime. The downside is that daily differential backups will take more time to carry out, and occupy more storage than incremental backups.
Synthetic full backup
A synthetic backup combines the last full backup with subsequent incrementals to provide a full that is always up to date. Synthetic full backups are easy to restore from, but also do not overly tax the network during the backup itself because only changes are transmitted. There is, however, a processing overhead at the backup server.
Incremental forever and reverse incremental
Variants built on fulls and incrementals include incremental forever and reverse incremental.
Incremental forever backups retain fulls and subsequent incrementals, so that restores can be to chosen points in time.
Meanwhile, reverse incremental backups are where a synthetic full backup is the default, but incrementals are retained to allow roll-back to a specified point.