Virtual Machine Disk Images

A virtual disk image is a file on a physical disk, which has a well-defned, published or proprietary, format and is interpreted by a Virtual Machine Monitor as a hard disk. 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, .vhd for Xen and Microsoft Hyper-V, .vdi for Oracle VM VirtualBox, etc.

Storage Allocation

There are two schemes pre-dominant across all Virtual Machine Monitor implementations, namely,

Pre-allocate the entire storage for the virtual disk upon creation, or,
Dynamically grow the storage on demand

Pre-Allocated 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 ^[1] 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 ^[1] 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 ^[2] ^[1]	Import	Native	Native	Native	Native	Native (SCSI)
VMware Workstation ^[3]	Native	Native		Native	Native	Native (IDE)
VMware Fusion ^[4]	Native	Native		Native	Native
VMware Server ^[5]	Native	Native		Native	Native
Xen ^[6]	Native	Native	Native	Native	Native
Microsoft Windows Server 2008 R2 ^[7]		Native		Native		Native
Microsoft Virtual PC 2007 ^[8]		Native		Native
Microsoft Virtual Server 2005		Native		Native
VirtualBox ^[9]	Import	Native	Import	Native		Native
Parallels Desktop ^[10]	Native	Native	Native	Native
Parallels Workstation ^[11]	Native	Native	Native	Native
Parallels Server Bare Metal ^[12]	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	VHD	Virtual Desktop Image	Oracle VM Image	Parallels hdd	qcow qcow2
VMware ESX Server	Native	Import
VMware Workstation	Native
VMware Fusion	Native
VMware Server	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
Oracle VM	Import	Import		Native
Parallels Desktop					Native
Parallels Workstation					Native
Parallels Server Bare Metal					Native
QEMU	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 ^[1] for VMware ESX, vmware-viskmanager ^[13] for VMware Workstation, Fusion and Server; vboxmanage ^[9] for VirtualBox, or,
Discrete tools supporting one or more operations on virtual disk files of a given format, e.g., VHD Tool^[14], or across one or more product lines, e.g., SCVMM ^[15], 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 ^[2] ^[1]	vmkfstools ^[1]	vmkfstools ^[1]		vmkfstools ^[1]				vmotion ^[16]
VMware Workstation ^[3]	vdiskmanager ^[13]		vdiskmanager ^[13]	vdiskmanager ^[13]	vdiskmanager ^[13] fatVM ^[17]	vdiskmanager ^[13]	vdiskmanager ^[13] contig ^[18]
VMware Fusion ^[4]
VMware Server ^[5]
Xen ^[6]
Microsoft Windows Server 2008 R2 ^[7]	Hyper-V Manager ^[19]	Hyper-V Manager ^[19] SCVMM ^[15]		Sysprep ^[20]	Hyper-V Manager ^[19]	Hyper-V Manager ^[19] SCVMM ^[15]
Microsoft Virtual PC 2007 ^[8]	vhdtool ^[14]	SCVMM ^[15] vmdk2vhd ^[21] StarWind ^[22] WinImage ^[23] SmartVDK ^[24]			vhdtool ^[14] VHD Utility ^[25] VHD Resizer ^[26] EASEUS ^[27] eXPander ^[28]	SCVMM ^[15] VHD Resizer ^[26]	contig ^[18]
Microsoft Virtual Server 2005	vhdtool ^[14]					SCVMM ^[15] VHD Resizer ^[26]	contig ^[18]
VirtualBox ^[9]	vboxmanage ^[9]	vboxmanage ^[9]	vboxmanage ^[9]	vboxmanage ^[9]		vboxmanage ^[9]	contig ^[18]	vboxmanage ^[9]
Parallels Desktop ^[10]	File Menu ^[10]	Transporter ^[29]	Transporter ^[29]	Clone VM Assistant ^[10]	Image Tool ^[30]		Compressor ^[31]
Parallels Workstation ^[11]	File Menu ^[11]			Clone VM Wizard ^[11]		Image Tool ^[30]
Parallels Server Bare Metal ^[12]	New VM Assistant ^[12]			Clone VM Assistant ^[12]		Image Tool ^[30]

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.

References

^ ^a ^b ^c ^d ^e ^f ^g ^h [1], VMWare ESX Configuration Guide
^ ^a ^b [2],VMware vSphere Basic System Administration.
^ ^a ^b [3], VMware Workstation User's Manual
^ ^a ^b [4], Getting Started with VMware Fusion.
^ ^a ^b [5], VMware Server User's Guide.
^ ^a ^b [6],Xen Users' Manual.
^ ^a ^b [7], Storage options for Windows Server 2008 Hyper-V.
^ ^a ^b [8], VIrtual hard Disk Image Format Specification. Cite error: The named reference "msftvhd" was defined multiple times with different content (see the help page).
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ [9], VirtualBox User Manual. Cite error: The named reference "vbox" was defined multiple times with different content (see the help page).
^ ^a ^b ^c ^d [10], Parallels Desktop User's Guide.
^ ^a ^b ^c ^d [11], Parallels Workstation User's Guide.
^ ^a ^b ^c ^d [12], Parallels Server Bare Metal User's Guide.
^ ^a ^b ^c ^d ^e ^f ^g [13], VMWare Virtual Disk Manager User's Guide
^ ^a ^b ^c [14],VHD Tool
^ ^a ^b ^c ^d ^e [15], System Center Virtual Machine Manager Cite error: The named reference "scvmm" was defined multiple times with different content (see the help page).
^ [16], VMware VMotion datasheet
^ [17], fatVM
^ ^a ^b ^c [http://technet.microsoft.com/en-us/sysinternals/bb897428.aspx, Windows Sysinternals Contig Cite error: The named reference "contig" was defined multiple times with different content (see the help page).
^ ^a ^b ^c ^d [18], Hyper-V Manager. Cite error: The named reference "hypervmgr" was defined multiple times with different content (see the help page).
^ [19], How to use Sysprep: An Introduction.
^ [20], VMDK(VMware) to VHD Converter
^ [21], StarWind V2V Converter
^ [22], WinImage
^ [23], SmartDeploy
^ [24], VHD Utility
^ ^a ^b [25], VHD Resizer
^ [26], EASEUS Partition Master
^ [27], eXPander
^ ^a ^b [28], Parallels Transporter User Guide.
^ ^a ^b [29], Parallels Image Tool User's Guide.
^ [30], Parallels Compressor User's Guide.

[vmwesx-1] ^ ^a ^b ^c ^d ^e ^f ^g ^h [1], VMWare ESX Configuration Guide

[vmwvbsa-2] [2],VMware vSphere Basic System Administration.

[vmwks-3] [3], VMware Workstation User's Manual

[vmwfus-4] [4], Getting Started with VMware Fusion.

[vmwsvr-5] [5], VMware Server User's Guide.

[xensrc-6] [6],Xen Users' Manual.

[msfthyperv-7] [7], Storage options for Windows Server 2008 Hyper-V.

[msftvhd-8] [8], VIrtual hard Disk Image Format Specification. Cite error: The named reference "msftvhd" was defined multiple times with different content (see the help page).

[vbox-9] ^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ [9], VirtualBox User Manual. Cite error: The named reference "vbox" was defined multiple times with different content (see the help page).

[pardtop-10] [10], Parallels Desktop User's Guide.

[parwks-11] [11], Parallels Workstation User's Guide.

[parsvr-12] [12], Parallels Server Bare Metal User's Guide.

[vmwvdm-13] ^ ^a ^b ^c ^d ^e ^f ^g [13], VMWare Virtual Disk Manager User's Guide

[vhdtool-14] [14],VHD Tool

[scvmm-15] [15], System Center Virtual Machine Manager Cite error: The named reference "scvmm" was defined multiple times with different content (see the help page).

[vmotion-16] [16], VMware VMotion datasheet

[fatvm-17] [17], fatVM

[contig-18] [http://technet.microsoft.com/en-us/sysinternals/bb897428.aspx, Windows Sysinternals Contig Cite error: The named reference "contig" was defined multiple times with different content (see the help page).

[hypervmgr-19] [18], Hyper-V Manager. Cite error: The named reference "hypervmgr" was defined multiple times with different content (see the help page).

[sysprep-20] [19], How to use Sysprep: An Introduction.

[vmdk2vhd-21] [20], VMDK(VMware) to VHD Converter

[starwind-22] [21], StarWind V2V Converter

[winimage-23] [22], WinImage

[smartvdk-24] [23], SmartDeploy

[vhdutil-25] [24], VHD Utility

[vhdrsz-26] [25], VHD Resizer

[easeus-27] [26], EASEUS Partition Master

[expander-28] [27], eXPander

[partran-29] [28], Parallels Transporter User Guide.

[parimg-30] [29], Parallels Image Tool User's Guide.

[parcomp-31] [30], Parallels Compressor User's Guide.

[1]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14]

[15]

[16]

[17]

[18]

[19]

[20]

[21]

[22]

[23]

[24]

[25]

[26]

[27]

[28]

[29]

[30]

[31]