Specifications

Best Practices for Virtualizing and Managing Exchange 2013

VHDX File Format

Hyper-V in Windows Server 2012 introduces VHDX, a new version of the virtual hard disk format that is

designed to handle current and future workloads. VHDX has a much larger storage capacity than the older

VHD format. It also provides protection from data corruption during power failures and optimizes

structural alignments to prevent performance degradation on new, large sector physical disks. The main

features of the VHDX format are as follows:

 Support for virtual hard disk storage capacity of up to 64 TB.

 Protection against data corruption during power failures by logging updates to the VHDX

metadata structures.

 Improved alignment of the virtual hard disk format to work well on large sector disks.

 Larger block sizes for dynamic and differencing disks, which allows these disks to attune to the

needs of the workload.

 A 4-KB logical sector virtual disk that allows for increased performance when used by applications

and workloads that are designed for 4-KB sectors.

 The ability to store custom metadata about a file that the user might want to record, such as

operating system version or patches applied.

 Efficiency in representing data (also known as trim), which results in smaller file size and allows

the underlying physical storage device to reclaim unused space. (Trim requires physical disks

directly attached to a virtual machine or SCSI disks, and trim-compatible hardware.)

Best Practices and Recommendations

When you create virtual machines on Windows Server 2012 Hyper-V, the VHDX file format should

be the default choice. While not compatible with previous versions of Hyper-V, its capacity

advantage, better alignment with underlying storage, and stronger protection against corruption

make VHDX an ideal choice for mission-critical workloads like Exchange 2013.

Dynamically Expanding VHDX vs. Fixed-Size VHDX

Fixed-size VHDX uses the full amount of space specified during virtual hard disk creation. However, the

size of a fixed-size VHDX can be increased while the virtual machine is offline by using Hyper-V Manager

or running a PowerShell script. Note that reducing the size is not supported. Fixed-size VHDX delivers

near native-to-physical performance and slightly higher performance than dynamically expanding VHDX

files.

During virtual hard disk creation, dynamically expanding VHDX files only consume physical space based

on their actual contents. For instance, an administrator could create a dynamically expanding VHDX with a

maximum size of 127 GB. Upon creation, the actual physical size of the VHDX file may only be a few MB,

but as files are added to the VHDX inside the guest operating system, the size of the VHDX file in the

physical world grows accordingly. The guest operating system always sees the maximum size that the

administrator chose upon creation.