If a backup job fails due to interrupted connection to the backup repository, Veeam Agent will perform a backup job retry. During retry, instead of restarting data transfer from the beginning, Veeam Agent will try to resume the backup process from the point where it was interrupted.
During the backup process, Veeam Agent flushes the data it copied from the source file system to the target storage at the default interval of 5 minutes. You can fine-tune the interval of the buffer cache flush in the Veeam Agent configuration file, but keep in mind that flushing buffer cache too often during data transfer may have a negative impact on the performance. For every file that is larger than 100 MB, Veeam Agent also creates a sync file that contains information on the amount of data copied from the file.
If the connection to the target repository is interrupted, after it is restored, Veeam Agent retries the backup job. Depending on the type of the source file system, Veeam Agent behaves differently:
- If the data to back up resides on a volume with an APFS file system, Veeam Agent will find the sync file for the file on which the backup process was interrupted. Veeam Agent will read the offset value from the sync file and will resume transferring data from the APFS snapshot to the target repository. After the file is fully copied to the target repository, Veeam Agent deletes its sync file.
If for some reason, after the connection to the backup repository is restored, the APFS snapshot is no longer available, Veeam Agent will trigger the creation of a new snapshot. Then Veeam Agent will compare the modification times of the files in the new snapshot and on the target repository. In the backup repository, Veeam Agent will replace all files that have mismatching modification timestamps with their latest versions from the newly created snapshot.
- If the data to back up resides on a volume with any supported file system excluding APFS, Veeam Agent performs the backup of such data in the snapshot-less mode. In this case, after the connection to the repository is restored, Veeam Agent will compare the modification times of the files in the source file system and in the target repository. In the backup repository, Veeam Agent will replace all files that have mismatching modification timestamps with their latest versions from the source file system.
Consider the following limitations of Resume Backup:
- Resume Backup is initiated during a retry of a scheduled backup job only. If you start the backup job manually and Veeam Agent loses connection to the target repository during backup, after the connection is restored, Veeam Agent will remove the incomplete restore point and create a new restore point.
- Resume Backup is disabled for backups to local and directly attached storage repositories.