Method for tracking changes in virtual disks

A computer program product comprising a computer readable medium encoded with a plurality of instructions for controlling a processor to perform an operation for tracking changes made to a virtual disk of a virtual machine, the instructions comprising:

• receiving a request to write data to a first virtual block of the virtual disk, wherein the virtual disk contains a plurality of virtual blocks whose data is stored in blocks of a storage subsystem;
• mapping a first write command to the first virtual block to a second write command to a first block of the storage subsystem;
• sending, from a virtualization software layer, the second write command to the storage subsystem;
• receiving, at the virtualization software layer, a write completion response from the storage subsystem;
• and
• based on the write completion response, maintaining tracking information in the virtualization software layer, wherein the tracking information indicates whether each of the plurality of virtual blocks has been written to since an event.

A method of backing up data on a storage device, the method comprising:

• incrementing a counter each time a backup of a plurality of blocks of a storage device is performed;
• tracking a first value of the counter when a first backup routine performs a backup of the storage device;
• tracking a second value of the counter when a second backup routine performs a backup of the storage device;
• for each block, storing the counter value of when data for a block was last changed;
• and
• performing a backup with the first backup routine by retrieving the data for each of the blocks that have a counter value higher than the first value for the last backup performed by the first backup routine.