If you perform image-level backups of Azure VMs or backups of Azure SQL databases, Veeam Backup for Microsoft Azure creates a new backup file in a backup repository during every backup session. A sequence of backup files created during a set of backup sessions makes up a backup chain.
The backup chain includes backup files of the following types:
- Full — a full backup file stores a copy of the full Azure VM or Azure SQL database image.
- Incremental — incremental backup files store incremental changes of Azure VM or SQL database images.
To create a backup chain for an Azure VM or an Azure SQL database protected by a backup policy, Veeam Backup for Microsoft Azure implements the forever forward incremental backup method:
- During the first backup session, Veeam Backup for Microsoft Azure copies the full Azure VM or SQL database image and creates a full backup file in a backup repository. The full backup file becomes a starting point of the backup chain.
- During subsequent backup sessions, Veeam Backup for Microsoft Azure copies only those data blocks that have changed since the previous backup session and stores these data blocks to an incremental backup file in a backup repository. The content of each incremental backup file depends on the content of the full backup file and the preceding incremental backup files in the backup chain.
Full and incremental backup files act as restore points for backed-up Azure VMs or Azure SQL databases that let you roll back instance data to the necessary state. To recover an Azure VM or Azure SQL database to a specific point in time, the chain of backup files created for the instance must contain a full backup file and a set of incremental backup files dependent on the full backup file.
If some file in the backup chain is missing, you will not be able to roll back to the necessary state. For this reason, you must not delete individual backup files from the backup repository manually. Instead, you must specify retention policy settings that will let you maintain the necessary number of backup files in the backup repository. For more information, see Performing Azure VM Backup (step 6) and Performing Azure SQL Backup (step 5).