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 OS reads from or writes to a hard disk, Oracle VM VirtualBox redirects the request to the image file.
Like a physical disk, a virtual disk has a size, or capacity, which must be specified when the image file is created. As opposed to a physical disk however, Oracle VM VirtualBox enables you to expand an image file after creation, even if it has data already. See Section 7.22, “VBoxManage modifymedium”.
Oracle VM VirtualBox supports the following types of disk image files:
VDI. Normally, Oracle VM VirtualBox uses its own container format for guest hard disks. This is called a Virtual Disk Image (VDI) file. This format is used when you create a new virtual machine with a new disk.
VMDK. Oracle VM VirtualBox also fully supports the popular and open VMDK container format that is used by many other virtualization products, such as VMware.
VHD. Oracle VM VirtualBox also fully supports the VHD format used by Microsoft.
HDD. Image files of Parallels version 2 (HDD format) are also supported.
Due to lack of documentation of the format, newer versions such as 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 capacity and format, as mentioned in Section 1.7, “Creating Your First Virtual Machine”, there are two options for creating a disk image: fixed-size or dynamically allocated.
Fixed-size. If you create a fixed-size image, an image file will be created on your host system which has roughly the same size as the virtual disk's capacity. So, for a 10 GB disk, you will have a 10 GB file. 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.
Dynamically allocated. For more flexible storage management, use a dynamically allocated image. This will initially be very small and not occupy any space for unused virtual disk sectors, but will grow every time a disk sector is written to for the first time, until the drive reaches the maximum capacity chosen when the drive was created. While this format takes less space initially, the fact that Oracle VM VirtualBox needs to expand the image file consumes additional computing resources, so until the disk file size has stabilized, write operations may be slower than with fixed size disks. However, after a time the rate of growth will slow and the average penalty for write operations will be negligible.