The physical hard disk that you connect to a virtual machine can also be a network-attached disk, such as a logical unit number LUN in a storage area network SAN.
Many organizations choose to store their enterprise workloads in virtual hard disks VHDs. Hyper-V supports two VHD file formats: the traditional. It also provides data corruption protection during power failures and optimizes structural alignments of dynamic and differencing disks to prevent performance degradation on new, large-sector physical hard disks. Ensure that the physical hard disk is available. The most important limitation for connecting a pass-through disk to a virtual machine is that it cannot be connected to the virtualization server or to other virtual machines at the same time.
The virtual machine must have exclusive access to the physical hard disk. Generation 1 and generation 2 virtual machines support pass-through disks. The following procedure uses a generation two virtual machine. Select the server running Hyper-V and the virtual machine that you will use to attach the pass-through hard disk. Click Physical hard disk , and select the pass-through disk from available physical hard disks.
Generation 1 and generation 2 virtual machines support converting pass-through disks to a virtual hard disk file. The following procedure details how to convert a pass-through disk to the. Select the server running Hyper-V that contains the virtual machine with the pass-through physical hard disk. Keep the default disk type, which is Dynamically expanding , and then click Next.
Click the name of the appropriate offline physical hard disk from the available physical hard disks, and then click Next. Next, go to the Hard Drive section, select the Physical Hard Disk option, and then select the hard disk that you want to use from the drop-down list. If you look at the next figure, you can see that my Hyper-V virtual machine is configured to use a physical hard disk. There are of course perfectly legitimate reasons why someone might want to bind a virtual machine to a passthrough disk.
Some people claim that passthrough disks perform better than virtual hard disks because there is less overhead. I have also heard of people using passthrough disks to keep data isolated to a very specific location. Regardless of any perceived benefits, there are some significant disadvantages to using passthrough disks. These VMs must typically be backed up at the guest level. Perhaps more importantly, passthrough disks cause mobility issues for the virtual machines that use them.
Hyper-V replication does not work with passthrough disks, nor does the Checkpoint feature. A VM that uses passthrough disks can be live migrated, but there are heavy restrictions limiting the use of the Live Migration feature. Even though passthrough disks are still an officially supported part of Hyper-V, I recommend using them only when absolutely necessary.
The limitations caused by their use typically outweigh any benefit that might be derived by using them. Fortunately, there is a way to reconfigure your virtual machines to use virtual hard disks instead of passthrough disks. The migration process is actually relatively easy to complete. However, you have to make sure that you have enough free space to create a virtual hard disk that is at least as large as the physical disk that you are replacing. The migration process is usually safe, but it is always important to have a backup in case anything goes wrong.
So with that said, begin the process by shutting down the virtual machine. You can see the available options in the figure below. Click Next and you will be asked whether you want to create a fixed-length, dynamically expanding, or differencing disk.
Choose the Dynamically Expanding option and click Next. In case you are wondering, the Fixed Size option does deliver better overall performance, but it can be difficult to correctly size a fixed-length disk. Another application writing to the pass-through disk during data migration can result in data corruption. All rights reserved. Account Settings Logout. All Files. Submit Search. Pass-through Disks In this topic: About pass-through disks Resizing the source of a pass-through disk About Pass-through Disks A pass-through disk is the term used to identify a physical disk that is not a member of a pool, but is used as a storage source in a virtual disk.
Using Pass-through Disks to Migrate Data Pass-through disks are most often used as a temporary measure to migrate data. Hosts can maintain access to current disk contents while the existing data is transferred to mirrored virtual disks.
Features such as caching, replication over IP, mirroring, and snapshot operations can be performed on existing data during the migration process. Important Notes: The sector size of a pass-through disk must match the sector size of the virtual disk in which it is used as a storage source. When a virtual disk created from a pass-through disk is deleted, this operation does not delete the data on the pass-through disk.
When creating virtual disks from pass-through disks for the purpose of migrating storage, remember that a pass-through disk cannot have a virtual disk already created from it in order to be included in the list of potential storage sources when creating virtual disks. A mirror using a pass-through disk can be added to a single virtual disk created from a pass-through as long as the mirror does not have a virtual disk created from it.
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