Virtual block devices

Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for virtual block storage. In one aspect, a method includes receiving a request to initialize a virtual machine, the virtual machine having an associated virtual block device; accessing a file map comprising a plurality of file map entries; determining file map entries corresponding to blocks of data allocated to the virtual block device and one or more files in which the blocks of data allocated to the virtual block device are stored; determining that a particular one of the blocks allocated to the virtual block device has been written to a new position not associated with the particular block in the file map; and updating the position associated with the particular block to the new position.