By Crystal / Last Updated April 8, 2022

What is fixed disk and dynamic disk

When you create a Hyper-V virtual machine, you are asked to choose the disk type. Mostly, you may choose fixed disk or dynamic disk as virtual machine storage. So what’s the difference between Hyper-V fixed VHD and dynamic VHD?

Fixed-size disks are file-based virtual hard disks (VHDs) that have the pre-allocated size on the physical volume. On the other hand, Dynamically expanding VHDs start with small size, usually only kilobytes, but expand as additional storage space is required.

hyper-v disk

Difference between fixed disk and dynamic disk

Before converting Hyper-V fixed disk to dynamic, you need to know the difference between them. So in this section, we will discuss the differences between the fixed disk and dynamic disk in detail.

Fixed Disk: This type of disk will be divided with a fixed size at the time of creation.

Pros: It usually provides better performance when running applications that require advanced disk activity. It won’t change size when data is deleted and added.
Cons: The disadvantage is that it takes longer to create large fixed disks and the extra disk space reserved for future growth cannot be used for other virtual machine files, which takes up real resources.

Dynamic Disk: It is small at the beginning of creation, but gradually get larger as data is added to it.

Pros: It’s flexible to use physical storage space. It is recommended for servers that do not contain applications that use disks intensively.
Cons: As the disk expands, the performance will be degraded and the speed of reading data will be slowed down. And it’s riskier to cause disk fragmentation.

If your virtual machine requires frequent disk activity but lack of capacity, you just expand Hyper-V disk size. But if you still want to convert Hyper-V disk, please refer to the next part that introduces the detailed steps to convert fixed disk to dynamic.

How to convert fixed disk to dynamic

You can convert Hyper-V fixed disk to dynamic disk through Hyper-V Manager, which allows you to utilize disk space flexibly and efficiently. If you want to convert dynamic to fixed VHD, the process is similar.

1. Access to Hyper-V Manager, make sure you have powered off the virtual machine.

2. Right-click the VM and select Settings. On the opened window, locate the virtual hard disk and select Edit.

3. Click Next to Choose Action, select Convert >> Next.

convert hyper-v disk

4. On Choose Disk Format, select VHD or VHDX >> Next. VHD format support 2TB size of Virtual Hard Disk and VHDX format support 64TB size of Virtual Hard Disk.

hyper-v disk format

5. On Choose Disk Type, you can convert the Fixed size disk to Dynamically expanding that provides better use of physical storage. The disk file created is small initially and changes as data is added.

convert fixed disk to dynamic

6. Specify a destination to store the dynamic virtual hard disk. Then review the information and click Finish to close the window. You can notice that the converted VHD becomes smaller than the previous one because of the “Dynamically expanding”.

But dynamic virtual hard disks are not recommended for virtual machines that run server workloads in a production environment. You need to make sure you have enough physical storage space and delete or merge all snapshots before the disk conversion, otherwise the conversion process may fail. So, it’s better to back up your virtual machine before converting Hyper-V disk to prevent data loss in the process.

How to backup Hyper-V VM before disk conversion

For enterprises, data security comes first. 60 percent of small businesses that lose data will shut down within six months. That’s according to new information gathered by Washington, DC-based research firm Clutch. Before you make big changes, backup is necessary for you that helps you to avoid human error, natural disaster, viruses & malware and etc.

With the built-in feature, you can export and import VM or use a professional tool to backup Hyper-V VM efficiently. Here, I will use AOMEI Cyber Backup to show the backup process. With this powerful backup software, it brings the following benefits for you.

Lower cost of storage and time: Automatic clean-ups, and incremental backup greatly reduce storage space and backup time.
Enterprise security: The most reliable infrastructure to protect your data - both your data is in transit or at rest, and meet the security needs of your business.
Automate Backup: Schedule backup plans to run the VM backup automatically.
Fast Recovery: Instantly recover entire VMs, files and objects from duplicated backups and easily restore VM to new host with one click.

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Hassle-free Hyper-V VM backup solution

1. Add device: Open AOMEI Cyber Backup and click Source Device >> Hyper-V >> Add Hyper-V to add a Hyper-V host. After that, bind the device.

add hyper-v host

2. Create Backup Task: Click Backup Task >> Create New Task to open the settings window. Select Hyper-V Backup.

hyper-v backup

Name: Select the virtual machines you want to backup. It allows you to backup unlimited VMs.

select hyper-v vm

Target: Both local path or network path are available to store the virtual machine backup files.

backup target

Schedule: Select backup methods as full or incremental backup and automate execution time as daily, weekly, monthly (by date / week).

schedule hyper-v backup

Backup Cleanup: Specify the retention policy that will delete the old backup files automatically.

hyper-v retention policy

3. Restore: After completing backups, you can easily recover the Hyper-V VM to the original host or to a new host.

✜ Restore to new location: reset the RAM and Vital Processor.

restore hyper-v


In this article, I have introduced the detailed process to convert fixed disk to dynamic in Hyper-V. Before you start disk conversion, remember to backup your important VM data and note the cautions when using dynamic VHD.