Cannot start Azure VM after resizing OS Disk

Filippo Zanetti 0

Hi to all,

I resized a OS Disk to my VM WSRV2016 from 127GB to 200GB, when it was in unallocated state.

After that, my VM cannot longer starts.

The error starting VM is:

Failed to start virtual machine 'VM_NAME*'. Error: Disk size reduction is not supported. Current size is 214748365312 bytes, requested size is 136365212160 bytes.

Can you help me?

Thanks in advance

Filippo

Akshay kumar Mandha 1,655 Reputation points Microsoft Vendor

2024-12-20T23:10:04.35+00:00

Hi Filippo Zanetti,
Welcome to the Microsoft Q&A Platform! Thank you for asking your question here
Based on your query, Robina provided answer.! Could you please review once If you have any further queries, please let us know we will help you as needed.!
Akshay kumar Mandha 1,655 Reputation points Microsoft Vendor

2024-12-23T18:26:53.0833333+00:00

Hi Filippo Zanetti,
As you mentioned on latest query you called for azure support team Thanks for letting us know. It's a good idea to contact Microsoft for clearer guidance for production env behavior. If you have any another query in future, please let us know tag me in comment we will provide you information as you needed.
Thank you.!

2 answers

Robina 260 Reputation points

2024-12-20T15:30:04.5266667+00:00
The issue you’re facing is due to an unsupported disk size reduction attempt. Virtual machine disks typically cannot be resized to a smaller capacity once they have been allocated.

Suggested Solutions:

Restore the Original Disk Size: If you accidentally tried to reduce the disk size, revert the disk to its original size of 214748365312 bytes. Check your platform's disk management options for resizing or undoing changes.

Create a New Disk:

Create a new virtual disk with the desired smaller size (136365212160 bytes).

Migrate your data from the old disk to the new disk using tools like dd or platform-specific migration utilities. Restore from Backup (If Available):

If you have a backup of the VM or disk, restore it to a new disk with the desired size.

Attach the restored disk to your VM.

Platform-Specific Recommendations:

For Azure: Use disk snapshots or images to create a new disk with the desired size.

Why Disk Reduction Fails:

Reducing a disk size may lead to data loss or corruption because it involves altering the structure of the disk. Most virtualization platforms explicitly prevent this for safety reasons.
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Filippo Zanetti 0 Reputation points

2024-12-20T17:43:23.9966667+00:00

Hi Robina, thanks for your answer.

Unfortunately I have no backup of this VM, but only an old snapshot.

This VM is an old Gen1 VM and his disks are stored in a Blob.

The strange thing is the size of VHD on the VM is 127GB, but into VHDS Blob is 200GB.

I'm not trying to reduce size of OS Disk, but only increase, after trying to expand disk from VM page, the error becomes.

Can you help me?

thanks

Filippo

Robina 260 Reputation points

2024-12-20T18:00:22.2733333+00:00

Thanks for the clarification! Based on what you've shared, it sounds like the issue stems from a mismatch between the VHD size in the Blob (200GB) and the size reflected on the VM (127GB). This can happen when the disk was resized, but the partition within the VM wasn't extended to match the new size.

Check the Disk Size in Azure Portal:

Make sure the VM's OS disk is properly resized in the Azure Portal. Even if the VHD in the Blob is showing 200GB, it's possible that the VM is still referencing the old 127GB size. You can manually update the disk size from the Azure Portal or Azure CLI:

az disk update --name <your_disk_name> --resource-group <your_resource_group> --size-gb 200

Make sure the VM's OS disk is properly resized in the Azure Portal. Even if the VHD in the Blob is showing 200GB, it's possible that the VM is still referencing the old 127GB size. You can manually update the disk size from the Azure Portal or Azure CLI:
az disk update --name <your_disk_name> --resource-group <your_resource_group> --size-gb 200

This ensures that Azure recognizes the full 200GB size.

Extend the Partition:

Inside the VM, the partition may not have been extended to utilize the additional space (from 127GB to 200GB). You’ll need to extend the partition to recognize the full disk size.

Steps to extend the partition:

Boot the VM using a recovery disk or Windows PE.

Use Diskpart to check the disk and partition sizes:

diskpart list disk select disk 0 list partition select partition 1 extend

After extending the partition, restart the VM and check if it recognizes the full 200GB.

Use Azure Snapshots:

Since you have an old snapshot, you can try creating a new disk from that snapshot. This approach will restore the state of your VM at the time of the snapshot and may resolve any issues with the resizing.

You can create a new disk from a snapshot via Azure CLI:
az snapshot create --resource-group <your_resource_group> --name <snapshot_name> --

Then, attach this new disk to your VM and see if it resolves the issue.

Reattach the VHD:

Detach the existing disk from the VM, then reattach it as the OS disk. This can sometimes help the VM recognize the new disk size.

Check Compatibility with Gen1 VM:

As you mentioned, this is an old Gen1 VM. Gen1 VMs may have limitations when it comes to disk resizing or may require additional steps to fully recognize expanded disk sizes. Ensure that the VM supports the disk size you're trying to configure, or consider migrating to a Gen2 VM if feasible.

Why This Happens:

It can happen when the VM is unable to recognize the new disk size after an expansion, especially if the partition inside the OS wasn’t extended to match the new disk capacity. It’s important to extend both the disk and the partition.
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Filippo Zanetti 0 Reputation points

2024-12-20T23:16:14.63+00:00

Hi Robina,

because this VM is in production, I called Azure Support.

Applying this command, the VM was restarted:

az vm update -n "VM_NAME" -g "RG_NAME"

Thanks for your support!

Filippo
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