Planning for Disks and Storage

Applies To: Windows Server 2008, Windows Server 2008 R2

This section describes the storage configuration options that a server running Hyper-V supports. It also provides information that will help you plan your storage requirements. For step-by-step procedures that explain how to create virtual hard disks, attach physical disks, and configure iSCSI-based storage, see Configuring Disks and Storage.

You can use the following types of physical storage with a server that runs Hyper-V:

• Direct-attached storage (storage attached to the management operating system). You can use Serial Advanced Technology Attachment (SATA), external Serial Advanced Technology Attachment (eSATA), Parallel Advanced Technology Attachment (PATA), Serial Attached SCSI (SAS), SCSI, USB, and Firewire.

• Storage area networks (SANs). You can use Internet SCSI (iSCSI), Fibre Channel, and SAS technologies.

For more information about the requirements and other considerations about hardware, see Hardware Considerations for Hyper-V in Windows Server 2008.

Determining your storage configuration options on the management operating system

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.

• 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.

Determining your storage configuration options on virtual machines

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.

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: