Various embodiments of a system and method for backing up data from a plurality of backup server computers in a first backup storage tier to a backup server computer in a second backup storage tier are disclosed. According to one embodiment of the method, a group of backup data sources may be associated with a writer on the backup server computer. Each backup data source may comprise data to be backed up from one of the backup server computer systems in the first backup storage tier. The writer may keep track of the write speed at which data from the group of backup data sources is written to a target storage device, and the number of backup data sources in the group may be automatically adjusted based on the write speed.