Sun Microsystems VIRTUALBOX VERSION 3.1.0_BETA2 - Manual (Page 79)

5 Virtual storage
5.2 Disk image files (VDI, VMDK, VHD, HDD)
Disk image files reside on the host system and are seen by the guest systems as hard
disks of a certain geometry. When a guest operating system reads from or writes to a
hard disk, VirtualBox redirects the request to the image file.
Note that when you create an image file, its size needs to be specified, which rep-
resents a fixed geometry of the virtual disk. It is therefore not possible to change the
size of the virtual hard disk later.
VirtualBox supports four variants of disk image files:
• Normally, VirtualBox uses its own container format for guest hard disks – Virtual
Disk Image (VDI) files. In particular, this format will be used when you create a
new virtual machine with a new disk.
• VirtualBox also fully supports the popular and open VMDK container format that
is used by many other virtualization products, in particular, by VMware. 3
• VirtualBox also fully supports the VHD format used by Microsoft.
• Image files of Parallels version 2 (HDD format) are also supported. 4 For lack of
documentation of the format, newer formats (3 and 4) are not supported. You
can however convert such image files to version 2 format using tools provided
by Parallels.
Irrespective of the disk format, as briefly mentioned in chapter 1.6 , Creating your
first virtual machine , page 17 , there are two options of how to create a disk image:
fixed-size or dynamically expanding.
• If you create a fixed-size image of e.g. 10 GB, an image file of roughly the same
size will be created on your host system. Note that the creation of a fixed-size
image can take a long time depending on the size of the image and the write
performance of your hard disk.
• For more flexible storage management, use a dynamically expanding image.
This will initially be very small and not occupy any space for unused virtual disk
sectors, but the image file will grow every time a disk sector is written to for the
first time. While this format takes less space initially, the fact that VirtualBox
needs to constantly expand the image file consumes additional computing re-
sources, so until the disk has fully expanded, write operations are slower than
with fixed size disks. However, after a dynamic disk has fully expanded, the
performance penalty for read and write operations is negligible.
3 Initial support for VMDK was added with VirtualBox 1.4; since version 2.1, VirtualBox supports VMDK
fully, meaning that you can create snapshots and use all the other advanced features described above for
VDI images with VMDK also.
4 Support was added with VirtualBox 3.1.
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