We’re encountering an issue while using Packer to build and capture a VHD image in Azure.

Hi Dulith Kavinda Silva,

Welcome to Microsoft Q&A and thank you for posting your query here!

Your issue where the Packer build process fails during VHD capture with the error:

"failed to capture OS Disk with err: failed to copy to storage with err: error copying: waiting for file to copy: context deadline exceeded" is typically caused by the operation exceeding the built-in timeout while copying and capturing the OS disk image to Azure Storage. This can happen due to large disk sizes, network throughput constraints, or VM resource limitations.

Here are step-by-step suggestions to identify root causes and resolve this issue:

1. Review and Increase Timeout Settings:

By default, Packer has a timeout for image capture operations. Since you already increased it up to 3 hours, verify that it is properly set in your packer template under the custom_image_capture_timeout or related timeout properties. Sometimes, setting the timeout higher than default (e.g., 4+ hours) can help.

2. Use a Larger VM Size for Image Build:

The VM size you use for the build impacts performance. A Standard VM with low CPU/memory can cause slow disk IO and network throughput, resulting in timeout issues during capture.

Consider using a larger VM SKU with more memory and CPU, e.g., Standard_D4s_v3 or better.

This reduces bottlenecks when copying large VHD files to the storage account.

Documentation references similar guidance for Azure VM Image Builder performance tuning Troubleshoot Azure VM Image Builder

3. Optimize VHD Size Before Capture:

Reducing the size of the VHD can speed up the copy process and reduce chance of timeout.

Use tools inside the VM to zero free space (e.g., sdelete for Windows or dd on Linux).

Shrink partitions if possible.

Remove unnecessary files or logs before capture.

4. Use Premium or Standard SSD Storage:

Using a premium storage account improves disk throughput and can reduce copy delays.

Your testing with premium storage is on the right path.

Reference: Azure storage options considerations Azure Storage performance (general guidance)

5. Monitor and Troubleshoot Logs:

Review the Packer build logs carefully for any other warnings or errors around the time of failure.

Check the Azure Storage metrics for the target account to see if any throttling or network errors occurred.

Validate network connectivity and DNS resolution if using custom DNS (context deadline issues sometimes stem from network DNS failures) Context deadline exceeded troubleshooting

6. Retry Build After Cleaning Storage Container:

Sometimes partial VHD files remain in the storage container causing conflicts.

Manually delete any partial or zero-byte VHD blobs from the vhds container before starting a new build.

7. Additional Tips:

Ensure the disk is properly generalized if applicable (sysprep for Windows).

Confirm there is enough available space in the target storage account and region.

Check if your network has bandwidth or proxy restrictions affecting large file uploads.

i hope this helps, please do let me know if you have any questions on this!

Thanks,

Harish.

Hi Dulith Kavinda Silva,

Greetings for the day!

Just checking to see if you have a chance to check my previous response and helped, do let me know if you have any further questions on this.

Thanks,

Harish.

Hi Harish,

Thank you for your support. We are still trying the steps and will let you know if we need any further assistance.

Best regards,
Dulith Kavinda Silva

Hi,

I added the following setting to my Packer template:

"custom_image_capture_timeout": "4h"

However, the build failed with the following error:

Error: Failed to prepare build: "azure-arm"

1 error occurred:
    * unknown configuration key: '"custom_image_capture_timeout"'

Could you please help confirm if this parameter is still supported in the latest Packer Azure ARM builder, or if there’s an alternative way to extend the image capture timeout?

Thank you,