Volumes to be backed up share the same snapshot. The Agent has several criteria to determine which volume is suitable for snapshot placement.
Overall, the selection is done as follows:
1. Total amount of free space on all volumes that are included in a backup. Veeam will request 20% (can be edited in /etc/veeam.veeam.ini, storageFreePercent) of that value (204 GiB).
For instance, if sda and sdb are selected for backup and have 1024 GiB of free space in total, the agent will request 204 GiB.
2. After that veeam checks if the value is larger or equal to the default snapshot portion size (1GiB, can be edited in /etc/veeam.ini, portionSize)
3. After that veeam searches for volumes that can contain that much data.
Agent also performs some additional verification, e.g. if the selected snapshot storage is included in a backup, if there is a suitable filesystem (xfs, ext4, btrfs), and so on.