Various embodiments of a system and method for backing up data to a backup server computer are disclosed. According to one embodiment of the method, a group of backup data sources may be associated with a writer module on the backup server computer. Each backup data source may comprise data to be backed up from one of a plurality of client computer systems. The writer module may write the data from each of the backup data sources in the group to a target storage device in order to concurrently backup each backup data source to the target storage device. The writer module may also keep track of the rate at which data is written to the target storage device. The number of backup data sources in the group may be automatically adjusted based on the write rate, e.g., in order to maximize throughput to the target storage device.