1. A method for tracking changes made to a virtual disk of a virtual machine running on a computer system, the method comprising: receiving from a storage system a response acknowledging a completion of a write request to the virtual disk, wherein the write request was issued by the computer system on behalf of the virtual machine; based on block information corresponding to the response, updating tracking information maintained by a virtualization software layer of the computer system, wherein the tracking information indicates whether each of a plurality of blocks of the virtual disk has been written to since a backup event that backed up at least a portion of the virtual disk; upon an occurrence of each new backup event, incrementing an event counter, updating a log that maintains a corresponding event counter value for each block of the plurality of blocks, wherein the updating of the log comprises assigning the incremented event counter's value to the corresponding event counter value for each block that has been written to as indicated by the tracking information, and clearing the tracking information; receiving a request relating to performing a specific backup event by a first backup routine, wherein the request includes a stored event counter value indicating when the first backup routine last performed a backup of the virtual disk; and providing, in response to the request, identities of the plurality of blocks having a corresponding event counter value in the log that is higher than the stored even counter value in the request, thereby enabling the first backup routine to copy the identified blocks when performing the specific backup event.