The VMDK format includes multiple differing subformats, some of which store metadata in an external descriptor file, while others embed it with the main data in a single file.[9] A flat image allocates space ahead of time while a sparse image grows as the virtual machine writes to it. Flat images can use the underlying file system's sparse file capability, as is done with the vmfs format on ESXi. An image can also refer to a parent image and only store changes made in a copy-on-write fashion. This enables creating a snapshot of a virtual machine's state.
Extents
The descriptor specifies a series of one or more extents that typically refer to a file or device that holds the actual data, unless for example they are of type ZERO, which emulates a zero-filled extent.[10] Each extent can be marked either RW, RDONLY, or NOACCESS to signify that the virtual machine should have respectively read/write, read-only, or no access to that part of the disk. The number and types of extents in an image depend on its createType. An image with createType="custom" can contain an arbitrary combination of extents.
Virtual disk provisioning options
Flat disk images can be provisioned in one of three ways:
thin: Blocks are not allocated or zeroed during initial provisioning. Instead, the image is created as a sparse file. Block allocation and zeroing is performed at first access.
zeroedthick: Blocks are allocated during initial provisioning but are not zeroed until first access.
eagerzeroedthick: Blocks are allocated and zeroed during initial provisioning.
Convention: Any item in this table that has the form of "A+B" or "A+B+C" indicates a disk format that spans multiple files, where A contains the bulk of the data, and B and C are sidecar files.
