Where possible, Teradata ARC attempts to archive AMP data using sessions that are local to each of the AMPs. This minimizes the amount of data that must be transferred between nodes for the backup to complete.
When performing all-AMP backups, it is best to have sessions logged on to as many nodes as possible; this ensures that at least one session is local to each AMP, and increases the chance that the local session is used to archive the data.
When performing multi-stream backups, it is best to have each stream connect to only a subset of the nodes in the system. Teradata ARC automatically divides AMPs between the streams so that each stream receives AMPs that are local to its sessions. This improves the scalability of the solution, as well as further increases the chance of a local session being used to archive each AMP.
In order for this feature to work, Teradata ARC must be able to log on a MONITOR session to determine AMP location information. To grant this privilege, the following SQL can be used:
GRANT MONITOR TO <arcuser>;
where <arcuser> is the user account which ARC is using to perform the backup.