How Volume-Level Backup Works
During volume-level backup, Veeam Agent performs the following operations for every backup job session:
- When a new job session starts, Veeam Agent creates a backup file in the target location.
- In the backup file, Veeam Agent creates a disk for each backed-up disk. In disks, Veeam Agent creates blank partitions that have the same size and location as partitions in backed-up disks.
- Veeam Agent creates a snapshot of the volume whose data you want to back up. The snapshot is created on the volume that has enough free disk space to contain the snapshot data. To create a snapshot, Veeam Agent uses the Veeam Agent for Linux Driver.
The snapshot helps make sure that the data on the volume is consistent and does not change at the moment of backup. If a data block is about to change on disk during backup, Veeam Agent will copy this block to the snapshot. After the data block is overwritten on the source location, its original copy will remain intact in the snapshot.
Consider the following:
- [For incremental backup] Veeam Agent uses the Veeam Agent for Linux Driver to detect what blocks have changed on the volume since the previous job session. The driver keeps this information as a changed block tracking map in the RAM of your computer.
Mind that every time the driver is unloaded or the Veeam Agent computer is rebooted, the changed block tracking map is reset as well. In such case, to detect what data blocks have changed since the previous job session, Veeam Agent rescans the entire data added to the backup scope and creates a new changed block tracking map. In this case, backup requires more time.
To learn about full and incremental backup, see Backup Chain.
- Veeam Agent copies the partition table and bootloader located on the hard disk to the backup file in the target location.
- [For incremental backup] Veeam Agent calculates checksums for each data block and compares them with checksums from the backup file created during the previous job session. If checksums do not match, Veeam Agent will copy the data block to the target location during the next backup process step.
- Veeam Agent copies data from the following sources:
- Data that did not change on disk during backup is transferred from the source volume.
- Data that changed on disk during backup is transferred from the snapshot.
After all the data is transferred, Veeam Agent removes the snapshot.