Hyper-V Pass-through Disk: Pros and Cons

Pass-through disks in Hyper-V are touted to offer better performance; however, the speed advantage is rather minimal and below 5% compared to a fixed-sized VHDX.

When using pass-through disks one needs to be aware of a couple of disadvantages:

Exclusive Use

Pass-through disks are under exclusive use by the VM, even though the access is still not physical and direct, it’s still virtualized.

Exclusive use by the VM, however, implies the host can no longer use the disk as its own. VSS, the vital component for live backups, can also no longer control the volume from the host side; hence, all live backups must be taken from inside the VM instead, where VSS has proper (and exclusive) access to the pass-through disk.

Physical Dependence, Lack of Portability

Unlike a drug user who takes his drugs with him, many pass-through disk configurations do not offer portability. One exception are iSCSI attached pass-through disks which can remain connected when a VM migrates live from one CSV node to another. Because disk access is routed through the IP network and Hyper-V live migrations keep network connections intact, this type of pass-through does indeed work in CSVs.

However, physically attached disks mounted as pass-through into a VM do not offer that advantage and create a dependency to a particular node. For standalone Hyper-V hosts that’s not an issue but for CSV setups it’s crucial not to set thing up this way as it interferes with the VM’s capability to migrate.

No Changes Possible

On Windows Server 2012 you can live resize fixed sized VHDX without shutting down the VM. That’s a great feature and important alternative and reason to not use dynamic disks nor pass-through disks.

With pass-through disks in Hyper-V you cannot change their size at all.

Granular Backup is not Available

While Granular Hyper-V Backup is available and saves money over full Hyper-V backup, this critical feature cannot be offered on pass-through disks, again because exclusive access is not possible from the host. Installing backup software or agents inside the VM is costly and also increases ongoing management expenses, too. If you switch to a fixed-sized VHD instead, you could apply granular backup to pull certain folders and files from the VM and back them up more frequently than the rest of the VM. This is a good way to protect SQL databases and Exchange more efficiently.

Alternatives to Pass-Through Disks

If you already have pass-through disks set up, you could convert them, see this article for a Powershell example.

The best alternative is to use a fixed sized VHDX instead which would incur a performance drop of less than 5% but give you portability and flexibility that isn’t possible with pass-through disks. While it may look inefficient to place a single, huge VHDX inside a large disk, the benefits can be worth a lot. You can easily copy the VHDX file elsewhere, for example. Or if it’s on a cluster shared volume, the VM can migrate freely without losses or interruptions. On standalone servers you also benefit from being able to back up the disk directly and being able to perform shared-nothing live migrations.

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BackupChain Features

BackupChain is a Windows Server 2012 backup software that has also grown worldwide to be the IT pro’s choice for a customized cloud backup solution. VM backup and especially cluster shared volume backup are just some of the many Hyper-V features BackupChain has to offer. To backup Hyper-V files or restore a Hyper-V file backup, you could use BackupChain’s granular restore feature to access the Hyper-V file backup without actually restoring it in its entirety.
For smaller businesses it’s interesting to note that USB Backup and NAS Backup are cost-effective ways to protect SMB data, whether you are backing up an entire Windows Server or just performing a Hyper-V Linux backup, external storage points are gaining popularity and are fully supported by BackupChain, including file version backup on such devices.
Our latest freeware addition to our IT pro toolset is DriveMaker, a free WebDrive alternative and freeware for Windows, which allows you to connect to FTP cloud storage from a mounted virtual disk device, such as X:\.

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