Virtual Machine Storage Dynamically Expanding Virtual Hard Disks Dynamically expanding VHDs provide storage capacity as needed to store data. The size of the VHD file is small when the disk is created, and it grows as data is added to the disk. The size of the VHD file does not shrink automatically when data is deleted from the virtual hard disk; however, you can use the Edit Virtual Hard Disk Wizard to make the disk more compact and decrease the file size after data is deleted. Fixed-Size Disks Fixed-size VHDs provide storage capacity by using a VHDX file that is in the size that is specified for the virtual hard disk when the disk is created. The size of the VHDX file remains fixed, regardless of the amount of data that is stored. However, you can use the Edit Virtual Hard Disk Wizard to increase the size of the VHDX, which in turn increases the size of the VHDX file. By allocating the full capacity at the time of creation, fragmentation at the host level is not an issue. (Fragmentation inside the VHDX itself must be managed within the guest.) Differencing Virtual Hard Disks Differencing VHDs provide storage to help you make changes to a parent VHD without changing the disk. The size of the VHD file for a differencing disk grows as changes are stored to the disk. Pass-Through Disks Pass-through disks allow Hyper-V 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 (by using the GUID of the disk) without having to utilize the file system of the host. Given that the performance difference between fixed disk and pass-through disks is now negligible, the decision is based on manageability. For instance, a disk that is formatted with VHDX is hardly portable if the data on the volume will be very large (hundreds of gigabytes), given the extreme amounts of time it takes to copy. For a backup scheme with pass-through disks, the data can be backed up only from within the guest. When you are utilizing pass-through disks, no VHDX file is created because the LUN is used directly by the guest. Because there is no VHDX file, and there is no dynamic sizing or snapshot capability.
Virtual Machine Storage Dynamically Expanding Virtual Hard Disks Dynamically expanding VHDs provide storage capacity as needed to store data. The size of the VHD file is small when the disk is created, and it grows as data is added to the disk. The size of the VHD file does not shrink automatically when data is deleted from the virtual hard disk; however, you can use the Edit Virtual Hard Disk Wizard to make the disk more compact and decrease the file size after data is deleted. Fixed-Size Disks Fixed-size VHDs provide storage capacity by using a VHDX file that is in the size that is specified for the virtual hard disk when the disk is created. The size of the VHDX file remains fixed, regardless of the amount of data that is stored. However, you can use the Edit Virtual Hard Disk Wizard to increase the size of the VHDX, which in turn increases the size of the VHDX file. By allocating the full capacity at the time of creation, fragmentation at the host level is not an issue. (Fragmentation inside the VHDX itself must be managed within the guest.) Differencing Virtual Hard Disks Differencing VHDs provide storage to help you make changes to a parent VHD without changing the disk. The size of the VHD file for a differencing disk grows as changes are stored to the disk. Pass-Through Disks Pass-through disks allow Hyper-V 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 (by using the GUID of the disk) without having to utilize the file system of the host. Given that the performance difference between fixed disk and pass-through disks is now negligible, the decision is based on manageability. For instance, a disk that is formatted with VHDX is hardly portable if the data on the volume will be very large (hundreds of gigabytes), given the extreme amounts of time it takes to copy. For a backup scheme with pass-through disks, the data can be backed up only from within the guest. When you are utilizing pass-through disks, no VHDX file is created because the LUN is used directly by the guest. Because there is no VHDX file, and there is no dynamic sizing or snapshot capability.
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