Fix VHD parsing - Take 2. #364
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Back in the Microsoft Virtual PC days, a VHD file's size was based on "Disk Geometry" instead of simple byte count. However, this method restricts the sizes that the disk can be. So, Hyper-V just ignores the "Disk Geometry" fields and uses the VHD's "Current Size" field instead. Unfortunately, `qemu-img` still by default outputs VHDs using "Disk Geometry". This generally means it has to round up the size. To make matters worse, the new disk size is almost never a multiple of 1 MiB, which can prevent this disk from being uploaded to Azure. When Image Customizer uses `qemu-img`, it knows to pass an option that tells `qemu-img` to use "Current Size" instead of "Disk Geometry". Specifically, `-o force_size=on`. But if a user manually calls `qemu-img`, they might not know they need to do this. This change adds logic to reject input VHDs that use "Disk Geometry" instead of "Current Size".
vinceaperri
reviewed
Aug 11, 2025
vinceaperri
approved these changes
Aug 12, 2025
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Back in the Microsoft Virtual PC days, a VHD file's size was based on "Disk Geometry" instead of simple byte count. However, this method restricts the sizes that the disk can be. So, Hyper-V just ignores the "Disk Geometry" fields and uses the VHD's "Current Size" field instead. Unfortunately,
qemu-imgstill by default outputs VHDs using "Disk Geometry". This generally means it has to round up the size. To make matters worse, the new disk size is almost never a multiple of 1 MiB, which can prevent this disk from being uploaded to Azure.When Image Customizer uses
qemu-img, it knows to pass an option that tellsqemu-imgto use "Current Size" instead of "Disk Geometry". Specifically,-o force_size=on. But if a user manually callsqemu-img, they might not know they need to do this.This change adds logic to reject input VHDs that use "Disk Geometry" instead of "Current Size". This will make it easier to catch problematic disks earlier, instead of Image Customizer being blamed for the incorrect disk size.
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