At the beginning of a job, if no other jobs are running, Veeam Backup & Replication retrieves the actual free space of the assigned Scale-Out Backup Repository extent. It then estimates the size of the restore point to be created and subtracts that from the actual free space to generate an estimated free space value. All subsequent concurrently running jobs for that extent use that estimated free space to keep a running Estimated Free space. This system allows the software to prevent jobs from running that may fill the extent.
More Information: Veeam Backup & Replication User Guide > Backup Size Estimation
The estimated free space system is intended as a safeguard to prevent concurrently running jobs from filling the Scale-Out Backup Repository extents.
More information can be found in the Backup File Placement section of the Veeam Backup & Replication User Guide.
Since the actual free space is only retrieved when no active tasks are assigned to an extent, in environments where tasks are always running, the estimated free space may begin to deviate from the actual free space over a long enough time. This deviation is due to the slight differences between the estimated size and the actual size of restore points.
The software can be configured to recalculate the extent's estimated free space when a task starts and is assigned to that extent. (The recalculation can only occur if it has been more than 15 minutes since the last estimated free space recalculation.) The recalculated 'estimated free space' will be based on the actual free space minus the estimated restore point size of all active tasks for the extent.
To enable this alternate method of estimating free space, create the following registry value:
Server: Veeam Backup Server
Key: HKLM\SOFTWARE\Veeam\Veeam Backup and Replication\
Value Name: SobrForceExtentSpaceUpdate
Value Type: DWORD
Value Data: 1
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