We get lots of cool tricks with virtualization. Among them is the ability to change our minds about almost any provisioning decision. In this article, we’re going to examine Hyper-V’s ability to resize virtual hard disks. Both Hyper-V Server (2016) and Client Hyper-V (Windows 10) have this capability.
Requirements for Hyper-V Disk Resizing
If we only think of virtual hard disks as files, then we won’t have many requirements to worry about. We can grow both VHD and VHDX files easily. We can shrink VHDX files fairly easily. Shrinking VHD requires more effort. This article primarily focuses on growth operations, so I’ll wrap up with a link to a shrink how-to article.
You can resize any of Hyper-V’s three layout types (fixed, dynamically expanding, and differencing). However, you cannot resize an AVHDX file (a differencing disk automatically created by the checkpoint function).
If a virtual hard disk belongs to a virtual machine, the rules change a bit.
- If the virtual machine is Off, any of its disks can be resized (in accordance with the restrictions that we just mentioned)
- If the virtual machine is Saved or has checkpoints, none of its disks can be resized
- If the virtual machine is Running, then there are additional restrictions for resizing its virtual hard disks
Can I Resize a Hyper-V Virtual Machine’s Virtual Hard Disks Online?
A very important question: do you need to turn off a Hyper-V virtual machine to resize its virtual hard disks? The answer: sometimes.
- If the virtual disk in question is the VHD type, then no, it cannot be resized online.
- If the virtual disk in question belongs to the virtual IDE chain, then no, you cannot resize the virtual disk while the virtual machine is online.
- If the virtual disk in question belongs to the virtual SCSI chain, then yes, you can resize the virtual disk while the virtual machine is online.
Does Online VHDX Resize Work with Generation 1 Hyper-V VMs?
The generation of the virtual machine does not matter for virtual hard disk resizing. If the virtual disk is on the virtual SCSI chain, then you can resize it online.
Does Hyper-V Virtual Disk Resize Work with Linux Virtual Machines?
The guest operating system and file system do not matter. Different guest operating systems might react differently to a resize event, and the steps that you take for the guest’s file system will vary. However, the act of resizing the virtual disk does not change.
Do I Need to Connect the Virtual Disk to a Virtual Machine to Resize It?
Most guides show you how to use a virtual machine’s property sheet to resize a virtual hard disk. That might lead to the impression that you can only resize a virtual hard disk while a virtual machine owns it. Fortunately, you can easily resize a disconnected virtual disk. Both PowerShell and the GUI provide suitable methods.
How to Resize a Virtual Hard Disk with PowerShell
PowerShell is the preferred method for all virtual hard disk resize operations. It’s universal, flexible, scriptable, and, once you get the hang of it, much faster than the GUI.
The cmdlet to use is Resize-VHD. As of this writing, the documentation for that cmdlet says that it operates offline only. Ignore that. Resize-VHD works under the same restrictions outlined above.
Resize-VHD -Path '\\svstore01\VMs\Virtual Hard Disks\test.vhdx' -SizeBytes 30gb
The VHDX that I used in the sample began life at 20GB. Therefore, the above cmdlet will work as long as I did at least one of the following:
- Left it unconnected
- Connected it to the VM’s virtual SCSI controller
- Turned the connected VM off
Notice the gb suffix on the SizeBytes parameter. PowerShell natively provides that feature; the cmdlet itself has nothing to do with it. PowerShell will automatically translate suffixes as necessary. Be aware that 1kb equals 1,024, not 1,000 (although both b and B both mean “byte”).
Had I used a number for SizeBytes smaller than the current size of the virtual hard disk file, I might have had some trouble. Each VHDX has a specific minimum size dictated by the contents of the file. See the discussion on shrinking at the end of this article for more information. Quickly speaking, the output of Get-VHD includes a MinimumSize field that shows how far you shrink the disk without taking additional actions.
This cmdlet only affects the virtual hard disk’s size. It does not affect the contained file system(s). That’s a separate step.
How to Resize a Disconnected Virtual Hard Disk with Hyper-V Manager
Hyper-V Manager allows you to resize a virtual hard disk whether or not a virtual machine owns it.
- From the main screen of Hyper-V Manager, first, select a host in the left pane. All VHD/X actions are carried out by the hypervisor’s subsystems, even if the target virtual hard disk does not belong to a specific virtual machine. Ensure that you pick a host that can reach the VHD/X. If the file resides on SMB storage, delegation may be necessary.
- In the far right Actions pane, click Edit Disk.
- The first page is information. Click Next.
- Browse to (or type) the location of the disk to edit.
- The directions from this point are the same as for a connected disk, so go to the next section and pick up at step 6.
Note: Even though these directions specify disconnected virtual hard disks, they can be used on connected virtual disks. All of the rules mentioned earlier apply.
How to Resize a Virtual Machine’s Virtual Hard Disk with Hyper-V Manager
Hyper-V Manager can also resize virtual hard disks that are attached to virtual machines.
- If the virtual hard disk is attached to the VM’s virtual IDE controller, turn off the virtual machine. If the VM is saved, start it.
- Open the virtual machine’s Settings dialog.
- In the left pane, choose the virtual disk to resize.
- In the right pane, click the Edit button in the Media block.
- The wizard will start by displaying the location of the virtual hard disk file, but the page will be grayed out. Otherwise, it will look just like the screenshot from step 4 of the preceding section. Click Next.
- Choose to Expand or Shrink (VHDX only) the virtual hard disk. If the VM is off, you will see additional options. Choose the desired operation and click Next.
- If you chose Expand, it will show you the current size and give you a New Size field to fill in. It will display the maximum possible size for this VHD/X’s file type. All values are in GB, so you can only change in GB increments (use PowerShell if that’s not acceptable).
If you chose Shrink (VHDX only), it will show you the current size and give you a New Size field to fill in. It will display the minimum possible size for this file, based on the contents. All values are in GB, so you can only change in GB increments (use PowerShell if that’s not acceptable).
Enter the desired size and click Next.
- The wizard will show a summary screen. Review it to ensure accuracy. Click Finish when ready.
The wizard will show a progress bar. That might happen so briefly that you don’t see it, or it may take some time. The variance will depend on what you selected and the speed of your hardware. Growing fixed disks will take some time; shrinking disks usually happens almost instantaneously. Assuming that all is well, you’ll be quietly returned to the screen that you started on.
Following Up After a Virtual Hard Disk Resize Operation
When you grow a virtual hard disk, only the disk’s parameters change. Nothing happens to the file system(s) inside the VHD/X. For a growth operation, you’ll need to perform some additional action. For a Windows guest, that typically means using Disk Management to extend a partition:
Note: You might need to use the Rescan Disks operation on the Action menu to see the added space.
Of course, you could also create a new partition (or partitions) if you prefer.
I have not performed this operation on any Linux guests, so I can’t tell you exactly what to do. The operation will depend on the file system and the tools that you have available. You can probably determine what to do with a quick Internet search.
VHDX Shrink Operations
I didn’t talk much about shrink operations in this article. Shrinking requires you to prepare the contained file system(s) before you can do anything in Hyper-V. You might find that you can’t shrink a particular VHDX at all. Rather than muddle this article will all of the necessary information, I’m going to point you to an earlier article that I wrote on this subject. That article was written for 2012 R2, but nothing has changed since then.
What About VHD/VHDX Compact Operations?
I often see confusion between shrinking a VHD/VHDX and compacting a VHD/VHDX. These operations are unrelated. When we talk about resizing, then the proper term for reducing the size of a virtual hard disk is “shrink”. “Compact” refers to removing the zeroed blocks of a dynamically expanding VHD/VHDX so that it consumes less space on physical storage. Look for a forthcoming article on that topic.