To avoid overload of backup proxies and backup repositories, Veeam Backup & Replication allows you to limit the number of concurrent tasks performed on a backup proxy (either on-host or off-host) or targeted at a backup repository.
Before processing a new task, Veeam Backup & Replication detects what backup infrastructure components (backup proxies and repositories) will be involved. When a new job starts, Veeam Backup & Replication analyzes the list of VMs that the job includes, and creates a separate task for each disk of every VM to be processed. Tasks in the job can be processed in parallel (that is, VMs and VM disks within a single job can be processed simultaneously), optimizing your backup infrastructure performance and increasing the efficiency of resource usage.
To use this capability, proxy server(s) should meet system requirements – each task requires a single CPU core, so for two tasks to be processed in parallel, 2 cores is the recommended minimum. Parallel VM processing must also be enabled in Veeam Backup & Replication options.
Task limiting is performed by the Veeam Backup Service that is aware of all backup proxies and backup repositories connected to it, and settings specified for these backup proxies and repositories (namely, the number of allowed concurrent tasks). When a job starts, it informs the Veeam Backup Service about its task list and polls the service about allocated resources to its tasks at a 10 second interval after that. Before a new task targeted at a specific backup proxy or repository starts, the Veeam Backup Service checks the current workload (the number of tasks currently working with the proxy or repository) and the number of allowed tasks per this component. If this number is exceeded, a new task will not start until one of the currently running tasks is finished.