When you create an instance from a custom image, the instance size must meet the minimum system disk capacity declared by the image. The dashboard checks this requirement on the image card, image sharing page, and when you submit the creation request.
During instance creation, the dashboard checks the selected configuration against the minimum requirements of the selected image. If you choose an image that needs a larger disk and then reduce the configuration, the image may no longer be eligible. For example, if “Windows 2022” requires Disk ≥ 40 GB and the new configuration provides only 20 GB, the dashboard shows:
The current image “Windows 2022” requires at least Disk ≥ 40 GB. Adjust the configuration or choose a compatible image to continue.
This does not mean that the image was deleted or that the configuration was lost. The two selections are temporarily incompatible. You can continue in any of these ways:
- Increase the configuration: Set the system disk to the image minimum or higher. In this example, the system disk must be at least 40 GB.
- Switch images: Select Review compatible image to see images that work with the current configuration, then choose the system recommendation.
- Keep the current image: Dismiss the prompt and keep the image, then return to the instance size and increase the disk. The create action stays disabled until the conflict is resolved, so an instance that cannot boot is not submitted accidentally.
The Review compatible image dialog presents the current image and the system recommendation as two cards connected by an arrow. The current image is on the left and the recommendation is on the right. Select Switch to compatible image to apply the recommendation. The dashboard first looks for a compatible version from the same operating system and only falls back to another operating system when necessary.
Image requirements can include more than disk capacity. They may also specify CPU, memory, network speed, IP counts, supported instance sizes, regions, or availability domains. The warning lists the conditions that are currently unmet; the image details and server-side validation remain the complete source of truth.
An instance size includes more than CPU and memory. It also includes resources such as the system disk and network. Images have their own requirements for normal operation, so every image cannot run on every size.
For custom images, the platform checks all of the following:
- Whether the instance system disk meets the image's minimum capacity.
- Whether the instance size is in the range supported by the image.
- Whether the selected region and availability zone can access the image.
Together, these conditions determine whether an image is available. The minimum size is not an additional billing tier, and it does not require every instance to use the same configuration. It prevents you from creating an instance that cannot boot or be fully restored.
Custom images are created from the system disk of an existing instance. In addition to the files you can see, an image contains operating system files, partition layouts, file system metadata, and boot data. When you create a new instance, the platform must restore all of this content to the target system disk.
If the target system disk is smaller than the minimum capacity recorded by the image, there is not enough space for a complete restore. The instance might fail to boot, files might be missing, or a later resize might leave the instance in an inconsistent state. The platform therefore rejects sizes below the limit while you select the image or submit the request, instead of waiting for an asynchronous creation task to fail halfway through.
The minimum capacity is a lower bound, not a fixed value that you must use. A system disk exactly at the minimum is valid; a larger disk leaves more room for operating system updates and application data.
Instance system disk capacity >= Image minimum system disk capacityFor example, if an image requires a minimum system disk of 40 GB:
| Image minimum | Instance system disk | Result |
|---|---|---|
| 40 GB | 20 GB | Does not meet the requirement; the image cannot be used |
| 40 GB | 40 GB | Meets the requirement; the image can be used |
| 40 GB | 80 GB | Meets the requirement; the image can be used |
Equality satisfies the requirement. The minimum is the lower bound for using the image; it does not mean that the instance will use all of that space.
When a custom image is created from an instance, the platform records the minimum system disk requirement based on the source instance's system disk capacity. The requirement remains with the image when it is copied, shared, or imported into another space.
Image file size and minimum system disk capacity are different concepts:
| Item | Meaning | Use |
|---|---|---|
| Image size | The storage space currently occupied by the image data. | Estimate storage usage when importing or copying. |
| Minimum system disk capacity | The minimum system disk required to boot an instance from the image. | Determine whether an instance size can use the image. |
Therefore, an image file being smaller than a system disk does not mean that any disk below the minimum requirement can use the image.
- Open Compute > Instances and select Create instance.
- Select a region and availability zone, then select an instance size.
- Select Custom image as the image source and find the image you want to use.
- Check the minimum disk requirement shown below the image card and confirm that the instance system disk meets it.
- If you use a flexible size, increase the disk to the minimum or higher, then finish configuring the login method and network.
An image can also be limited by supported instance sizes and availability zones. If the disk capacity meets the requirement but the image is still unavailable, choose a supported size or the availability zone where the image is available.
After you open a shared image link, the page displays the image's minimum configuration near the import button. Select the target region and availability zone during import. After the import completes, creating an instance still requires a size that meets the same minimum system disk requirement.
An enabled import button does not mean that every instance size can use the image. When you create the instance, both the dashboard and the service validate the system disk capacity and the image's supported sizes again.
The current instance system disk is smaller than the image requirement. Increase the disk for a flexible size, or choose a fixed size with a larger system disk before selecting the image.
In addition to disk capacity, an image might require a specific instance size, region, or availability zone. Confirm the deployment location first, then check whether the size is supported by the image.
Changing the size can change the available disk capacity or supported size range. Select the size again, then choose an image that meets the requirements from the custom image list.