Systems and methods for performing backups to a storage device are provided. For virtual disks of a virtual machine, snapshots are used to backup data periodically to a storage device. A disk virtualization layer “retires” data blocks associated with a snapshot, while retaining a list of block addresses, for comparison in future backup operations. The retired snapshot can be compared against future snapshots to generate incremental backups without occupying storage space with data blocks that have already been copied to another storage device.