A closer look at virtual hard disks in Windows Server 2012 R2

Stephen J. Bigelow

What is a virtual disk and what are the characteristics of a virtual disk under Windows Server 2012 R2?

A virtual disk is essentially a single file that is placed on a physical hard drive. The virtual disk file is designed to capture the complete state of the virtual machine residing in server memory and represent that content in an established disk file format. Windows Server 2012 R2 and Hyper-V employs the extended virtual hard disk (VHDX) file format, so files of virtual hard disks can be identified with a .VHDX extension.

Virtual hard disks are critical to virtualization. When a server powers on, each VM is loaded into server memory and launched from its corresponding VHDX file. As the VM operates, the VHDX file can be updated to reflect data or state changes. The VHDX file can be copied to remote storage to provide a backup or disaster recovery copy of the VM. A VHDX file can also be migrated or copied to other servers, allowing VMs to migrate or duplicate within the environment (as software licensing permits). Virtual hard disks are also well suited for centralized storage (rather than existing on each local server).

The virtual hard disk format has evolved to reflect changing needs of virtual machines and data center resources. One notable change is the increase in VHDX size from 2 TB to 64 TB, allowing for enormous virtual machines and data elements. This can make virtual disks better suited for applications such as databases or in-memory analytics.

Even with well-designed, redundant, battery and backup power, the unexpected loss of utility power always carries the potential for server crashes that can corrupt storage data -- especially data that changes regularly, such as virtual disk files. The VHDX format now logs all changes to the VHDX metadata. Windows Server 2012 R2 allows the creation of differencing disks so that one VHDX file can record the changes that take place on another. This tracks all changes so that unwanted or problematic changes can be reversed. This combination of capabilities allows the virtual server to recover its VMs with little (if any) data or state loss.

The VHDX format also introduces a variety of performance-enhancing features. For example, the VHDX file can be aligned with sectors on the physical hard drive to optimize performance on large drives, and file data can be represented more efficiently for smaller effective file sizes.

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