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Fixed Size Disks Fixed virtual hard disks provide storage capacity by using a .vhd file that is in the

size specified for the virtual hard disk when the disk is created. The size of the .vhd file remains 'fixed' regardless of the amount of data stored. However, you can use the Edit Virtual Hard Disk Wizard to increase the size of the virtual hard disk, which increases the size of the .vhd file. By allocating the full capacity at the time of creation, fragmentation at the host level is not an issue (fragmentation inside the VHD itself must be managed within the guest).

Differencing Disks Differencing virtual hard disks provide storage to enable you to make changes to

a parent virtual hard disk without altering that disk. The size of the .vhd file for a differencing disk grows as changes are stored to the disk.


For production environments, utilize Fixed Size disks, which provide better performance and ease the monitoring of storage availability. Utilizing fixed disks allocates the full size of the disk upon creation.

In Hyper-V R2, the performance of Dynamically Expanding disks (including Snapshots / .AVHDs, and Differencing Disks) has increased dramatically and are now viable options for production use. However, they carry other risks such as storage oversubscription and fragmentation, so use with caution.

Pass-Through Disks Hyper-V enables virtual machine guests to directly access local disks or SAN

LUNs that are attached to the physical server without requiring the volume to be presented to the host server. The virtual machine guest accesses the disk directly (utilizing the disk‘s GUID) without having to utilize the host‘s file system. Given

that the performance difference between Fixed-Disk and Pass-through Disks is now negligible, the decision is now based on manageability. For instance, if the data on the volume will be very large (hundreds of gigabytes), a VHD is hardly portable at that size given the extreme amounts of time it takes to copy. Also, bear in mind the backup scheme. With pass-through disks, the data can only be backed up from within the Guest.

When utilizing pass-through disks, there is no VHD file created; the LUN is used directly by the guest. Since there is no VHD file, there is no dynamic sizing capability or snapshot capability.


Use pass-through disks only in cases where absolute maximum performance is required and the loss of features such as snapshots and

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