1. A method for backing up virtual machine (VM) data, the method comprising: in response to a request to back up a first VM of a first storage system, determining a disk layout of a VM disk associated with the first VM to be backed up, the disk layout having a plurality of nodes in a hierarchical structure, each node corresponding one of a plurality of VMs of the VM disk; traversing the disk layout to derive a first storage location of a first VM configuration data of the first VM and to derive a second storage location of a second VM configuration data associated with a second VM, wherein the second VM is a parent VM to the first VM represented in the hierarchical structure; identifying a first VM snapshot from the first configuration data and a second VM snapshot from the second configuration data; determining a list of changed blocks representing data blocks that are different between the first VM snapshot and the second VM snapshot; and backing up the data blocks identified in the list of changed blocks from the first storage system to a second storage system.