A virtual disk image is a file on a physical disk, which has a well-defined, published or proprietary, format and is interpreted by a Virtual Machine Monitor

Virtual machine

A virtual machine is a "completely isolated guest operating system installation within a normal host operating system". Modern virtual machines are implemented with either software emulation or hardware virtualization or both together.-VM Definitions:A virtual machine is a software...

 as a hard disk. IT administrators and software developers administer them through offline operations using built-in or third-party tools. In terms of naming, a virtual disk image for a certain Virtual Machine monitor has a specific file type extension, e.g., .vmdk for VMware VMDK

VMDK

The VMDK file format is a type of virtual appliance developed for VMware products. The format is open/documented.-Products that use the format:* VMware Workstation* VMware Player* VMware Server* VMware Fusion...

, .vhd for Xen and Microsoft Hyper-V

VHD (file format)

A Virtual Hard Disk is a virtual hard disk file format, meaning it can contain what is found on a physical hard disk drive, such as disk partitions and a file system, which in turn can contain files and folders. It is typically used as the hard disk of a virtual machine...

, .vdi for Oracle VM VirtualBox, etc.

Storage allocation

There are two schemes predominant across all Virtual Machine Monitor implementations, namely,

1. Preallocate the entire storage for the virtual disk upon creation, or,
2. Dynamically grow the storage on demand

Preallocated storage

The virtual disk is implemented as either split over a collection of flat files, typically each one is 2GB in size, collectively called a split flat file, or as a single, large monolithic flat file. The pre-allocated storage scheme is also referred to as a thick provisioning scheme.

Dynamic storage growth

The virtual disk can again be implemented using split or monolithic files, except that storage is allocated on demand. Several Virtual Machine Monitor implementations initialize the storage with zeros before providing it to the virtual machine that is in operation. The dynamic growth storage scheme is also referred to as a thin provisioning scheme.

Raw disk mapping

There are two modes in which a raw disk can be mapped for use by a virtual machine

Virtual mode

The mapped disk is presented as if it is a logical volume, or a virtual disk file, to the guest operating system and its real hardware characteristics are hidden. In this mode, file locking provides data protection through isolation for concurrent updates; the copy on write operation enables snapshots. Virtual mode also offers portability across storage hardware because it presents the consistent behavior as a virtual disk file.

Physical mode

In this mode, also called the pass through mode, the Virtual Machine Monitor bypasses the I/O virtualization layer and passes all I/O commands directly to the device. All physical characteristics of the underlying hardware are exposed to the guest operating system. There is no file locking to provide data protection.

Survey of storage allocation policies and raw disk mapping support

This section intends to survey the storage allocation policies supported by several commercial virtual machine monitors for creating virtual disks, and the support for raw device mappings. "Native" indicates that the capability is natively built-in to the product, "Import" indicates that a format conversion is necessary before the Virtual Machine Monitor can employ it.

Operation	Create Virtual Disk and Allocate Storage				Raw Disk Mappings
Policy	Pre-allocate		Allocate On Demand
File / Disk Type	Split File Flat	Monolithic File Flat	Split File Sparse	Monolithic File Sparse	Virtual Mode	Physical Mode (Passthru)
Commercial Products
VMware ESX Server	Import	Native	Native	Native	Native	Native (SCSI)
VMware Workstation	Native	Native		Native	Native	Native (IDE)
VMware Fusion	Native	Native		Native	Native
VMware Server	Native	Native		Native	Native
Xen	Native	Native	Native	Native	Native
Microsoft Windows Server 2008 R2		Native		Native		Native
Microsoft Virtual PC 2007		Native		Native
Microsoft Virtual Server 2005		Native		Native
VirtualBox	Import	Native	Import	Native		Native
Parallels Desktop	Native	Native	Native	Native
Parallels Workstation	Native	Native	Native	Native
Parallels Server Bare Metal	Native	Native	Native	Native

Virtual disk formats

The following table provides a compatibility matrix that shows which particular virtual disk format is supported by a Virtual Machine Monitor. The term Native represents that the Virtual Machine Monitor inherently supports a particular virtual disk format, whereas, the term Import indicates that a conversion tool should be used to convert that format to another natively supported format.

Commercial Products	VMDK VMDK The VMDK file format is a type of virtual appliance developed for VMware products. The format is open/documented.-Products that use the format:* VMware Workstation* VMware Player* VMware Server* VMware Fusion...	VHD VHD (file format) A Virtual Hard Disk is a virtual hard disk file format, meaning it can contain what is found on a physical hard disk drive, such as disk partitions and a file system, which in turn can contain files and folders. It is typically used as the hard disk of a virtual machine...	Virtual Disk Image VirtualBox Oracle VM VirtualBox is an x86 virtualization software package, originally created by software company Innotek GmbH, purchased by Sun Microsystems, and now developed by Oracle Corporation as part of its family of virtualization products...	Oracle VM Image Oracle VM Oracle VM is the server virtualization offering from Oracle Corporation. Oracle VM is based on the open-source Xen hypervisor technology, supports Windows, Linux, and Oracle Solaris guests and includes an integrated Web based management console. Oracle VM features fully tested and certified Oracle...	Parallels hdd Parallels, Inc. Parallels, Inc. is a privately held virtualization technology company with offices in the USA, Germany, UK, France, Japan, China, Russia and Ukraine. Parallels' US offices are in Renton, WA.The company has more than 700 employees as of 2010....	qcow qcow2 Qcow qcow stands for "QEMU Copy On Write" and denotes a disk storage optimization strategy that delays allocation of storage until it is actually needed. QEMU is an emulator and virtual machine container, and it can use a variety of virtual disk images which are generally associated with specific guests...
VMware ESX Server	Native	Native
VMware Workstation	Native	Native
VMware Fusion	Native	Native
VMware Server	Native	Native
VMware Player	Native	Native
Xen	Native	Native
Microsoft Windows Server 2008 R2		Native
Microsoft Virtual PC 2007		Native
Microsoft Virtual Server 2005 R2		Native
Oracle VM VirtualBox	Native	Native	Native		Native (v.2)
Oracle VM	Import	Import		Native
Parallels Desktop					Native
Parallels Workstation					Native
Parallels Server Bare Metal					Native
QEMU QEMU QEMU is a processor emulator that relies on dynamic binary translation to achieve a reasonable speed while being easy to port on new host CPU architectures....	Native	Native	Native			Native

Note that QEMU is strictly not a virtual machine monitor. However, given that it is an integral component of key open source implementations, e.g., Xen, KVM, it is referenced here for completeness.

Offline virtual image operations and tools

This section surveys the tools offered by Virtual Machine Monitor and third-party vendors for some of the common operations performed on virtual disk images. The tools are offered either as,

• A packaged suite for a given product line, e.g. vmkfstools for VMware ESX, vmware-vdiskmanager for VMware Workstation, Fusion and Server; vboxmanage for VirtualBox, or,
• Discrete tools supporting one or more operations on virtual disk files of a given format, e.g., VHD Tool, or across one or more product lines, e.g., SCVMM , or,
• Built-in menu items in the user interface of the product, e.g., create (new) virtual disk, in VMware Workstation, Fusion, and Server; VirtualBox; Parallels Dekstop and Workstation.

Each tool requires that the virtual disk image should be offline, i.e., no virtual machine that uses this virtual disk image should be executing when the operation is performed.

Note that the extension / expansion / enlargement is meaningful only for those virtual disk images that have pre-allocated storage. The shrink and defragment operations are supported only for virtual disk images that grow dynamically by allocating storage on demand. The latter operations are performed on the disk blocks belonging to the virtual disk image; a subsequent OS-level defragmentation is necessary to return blocks freed by them to the unallocated list.

The table below lists the virtual disk image operations and the tools available for performing them per commercial product.

Virtual Disk Image Operation	Create	Convert (Import)	Export	Clone	Extend / Expand / Enlarge	Shrink	Defragment	Live Migrate
Disk Type	All	All	All	All	Pre-Allocated	Dynamic Growth	Dynamic Growth	All
Commercial Products
VMware ESX Server	vmkfstools	vmkfstools		vmkfstools				vmotion
VMware Workstation	vdiskmanager		vdiskmanager	vdiskmanager	vdiskmanager fatVM	vdiskmanager	vdiskmanager contig
VMware Fusion
VMware Server
Xen
Microsoft Windows Server 2008 R2	Hyper-V Manager	Hyper-V Manager SCVMM		Sysprep	Hyper-V Manager	Hyper-V Manager SCVMM
Microsoft Virtual PC 2007	vhdtool	SCVMM vmdk2vhd StarWind WinImage SmartVDK			vhdtool VHD Utility VHD Resizer EASEUS eXPander	SCVMM VHD Resizer	contig
Microsoft Virtual Server 2005
VirtualBox	vboxmanage	vboxmanage	vboxmanage	vboxmanage		vboxmanage	contig	vboxmanage
Parallels Desktop	File Menu	Transporter	Transporter	Clone VM Assistant	Image Tool		Compressor
Parallels Workstation	File Menu			Clone VM Wizard		Image Tool
Parallels Server Bare Metal	New VM Assistant			Clone VM Assistant
QEMU / KVM Kernel-based Virtual Machine Kernel-based Virtual Machine is a virtualization infrastructure for the Linux kernel. KVM supports native virtualization on processors with hardware virtualization extensions....	qemu-img create	virt-v2v		virt-clone	virt-resize	virt-resize		virsh migrate

There is a cluster of third-party tools available for Microsoft Virtual PC and Virtual Server product lines for converting non-Microsoft virtual disk image formats to the VHD format, and for resizing (expanding or shrinking) virtual disk images in the VHD format.