VHD Summary – Systems Engineering & Integration

A virtual hard disk (vhd) is a file that is created within the host operating system that simulates a hard disk for the virtual machine.

The virtual machine accesses the virtual hard disk through the management operating system.
When you create the virtual disk, you specify the disk and controller type (such as ATA/IDE or SCSI).
The virtual disk type does not have to match the physical disk type. For example, you can create a virtual SCSI disk on a physical ATA disk.
Do not create a virtual hard disk on a folder with encryption enabled. You can create a virtual hard disk on a volume with BitLocker enabled.

You have the following options for configuring a hard disk for use by the virtual machine:

Virtual hard disk format

The vhd format is the traditional format. The vhd format:

Supports files up to 2,040 GB in size.
Provides compatibility with previous versions of Windows.

The vhdx format is a new format introduced in Windows Server 2012. The vhdx format:

Supports virtual disks up to 64 TB in size.
Is more resilient during power failures.
Offers enhanced compatibility with large sector disks.
Is supported only in Windows Server 2012.

Virtual hard disk type

A fixed disk occupies a set amount of hard disk space in the management operating system. The size of the virtual hard disk file indicates the total storage capacity of the virtual disk.

This disk type takes longer to create than other disk types.
The entire disk size, including any empty space within the virtual hard disk, is reserved on the physical disk.
Performance is improved because the entire virtual disk is a contiguous block and because additional space does not need to be allocated to expand the size of the disk.

For best performance, use fixed disks for all virtual disks.

A dynamically expanding disk allocates physical disk space in the vhd file as virtual disk storage increases.

Physical disk space is allocated only as it is used by the virtual machine.
The size of the vhd file grows as more disk space is used by the virtual machine.
This disk type makes the most efficient use of hard disk space.
It is possible for the physical disk to run out of space as the vhd file size grows.

Keep in mind the following when creating a virtual hard disk:

After creating the virtual hard disk, you must initialize, partition, and format it.
The vhd detaches when you reboot the server.
You must manually reattach the vhd whenever you reboot or restart the server.
To manage a vhd file from the command line, use diskpart.
When creating a vhd file using diskpart, enter the vhd size in megabytes.