At the beginning of an online backup operation, the backup software system creates a snapshot of source data storage. The snapshot includes a watch-list used for identifying blocks of a source storage which are watched by snapshot management means for update. If a block included into the watch-list was requested for update, the snapshot management means preserve original contents of that block in a retention container for the purpose of temporary store. The retention container includes a set of temporal stores dedicated for transient storing of blocks until they are backed up. Backed up blocks can be operatively excluded from the snapshot so that unchanged blocks are excluded from the watch-list and updated blocks are removed from the retention container.