On the management operating system, you can select to use either virtual hard disks or physical disks that are directly attached to a virtual machine. Virtual hard disks can have a capacity of up to 2040 gigabytes and include the following types:

• Fixed size. A fixed size virtual hard disk is a disk that occupies physical disk space on the management operating system equal to the maximum size of the disk, regardless of whether a virtual machine requires the disk space. A fixed size virtual hard disk takes longer to create than other types of disks because the allocated size of the .vhd file is determined when it is created. This type of virtual hard disk provides improved performance compared to other types because fixed virtual hard disks are stored in a contiguous block on the management operating system.
  
  

  Important

  We recommend that you use fixed size virtual hard disks for virtual machines that are running on a production environment. Because a fixed amount of space on the underlying physical storage is allocated when you create a fixed size virtual hard disk, you can ensure that there will be enough storage space for all the virtual hard disks that you create. This will help you to avoid running out of storage space, as can happen when you use dynamically expanding or differencing virtual hard disks, which continue to grow as data is written to them.

• Dynamically expanding. A dynamically expanding virtual hard disk is a disk in which the size of the .vhd file grows as data is written to the disk. This type provides the most efficient use of disk space. You will need to monitor the available disk space to avoid running out of disk space on the management operating system.
  
  
• Differencing. A differencing virtual hard disk stores the differences from the virtual hard disk on the management operating system. This allows you to isolate changes to a virtual machine and keep a virtual hard disk in an unchanged state. The differencing disk on the management operating system can be shared with virtual machines and, as a best practice, must remain read-only. If it is not read-only, the virtual machine’s virtual hard disk will be invalidated.
  
  

With virtual hard disks, each virtual machine supports up to 512 TB of storage. Physical disks that are directly attached to a virtual machine have no size limit other than what is supported by the guest operating system.

Note
For more information about virtual hard disk performance and a comparison of performance in Windows Server 2008 and Windows Server 2008 R2, see Virtual Hard Disk Performance in the Microsoft Download Center (http://go.microsoft.com/fwlink/?LinkId=186519).

You can select either integrated device electronics (IDE) or SCSI devices on virtual machines:

• IDE devices. Hyper-V uses emulated devices with IDE controllers. You can have up to two IDE controllers with two disks on each controller. The startup disk (sometimes referred to as the boot disk) must be attached to one of the IDE devices. The startup disk can be either a virtual hard disk or a physical disk. Although a virtual machine must use an IDE device as the startup disk to start the guest operating system, you have many options to choose from when selecting the physical device that will provide the storage for the IDE device. For example, you can use any of the types of physical storage identified in the introduction section.
  
  
• SCSI devices. Each virtual machine supports up to 256 SCSI disks (four SCSI controllers with each controller supporting up to 64 disks). SCSI controllers use a type of device developed specifically for use with virtual machines and use the virtual machine bus to communicate. The virtual machine bus must be available when the guest operating system is started. Therefore, virtual hard disks attached to SCSI controllers cannot be used as startup disks.
  
  

Caution

By default, SCSI commands are filtered in Hyper-V in Windows Server 2008 R2. Although it is possible to disable filtering of SCSI commands, we do not recommend that you arbitrarily disable filtering because it represents a possible security risk (when filtering of SCSI commands is turned off, SCSI commands pass directly from the virtual machine to the storage stack on the management operating system). Programmatically disabling SCSI filtering is supported in Hyper-V in Windows Server 2008 R2 and may be required for certain storage management applications. You should only disable SCSI filtering after consulting with your storage vendor and determining that it is necessary for application compatibility.

Note
Although the I/O performance of physical SCSI and IDE devices can differ significantly, this is not true for the virtualized SCSI and IDE devices in Hyper-V. Hyper-V. IDE and SCSI devices both offer equally fast I/O performance when integration services are installed in the guest operating system.

The following table describes the various storage configuration options available with IDE devices:

The following table describes the various storage configuration options available with SCSI devices: