Support flattened convert in userspace convertor

[djdongjin]

What is the version of your Accelerated Container Image

No response

What would you like to be added?

Support generating a single layer overlaybd image in userspace convertor

Why is this needed for Accelerated Container Image?

Our production images have many layers (~70) and we're not using Layer/Manifest Deduplication.

Sometimes we observe long latency during rpull, and the latency is proportional to the image layers. (We suspect it's due to network issue when containerd/overlaybd talks to cloud provider during rpull to get layer metadata).

Currently our mitigation is to first flatten/squash the original image into a single layer image, and then convert that single layer image. This solves the long rpull issue.

It would be great if the userspace convertor can provide a feature that generates a single-layer overlaybd image directly.

cc @shuaichang

Are you willing to submit PRs to contribute to this feature?

• [ ] Yes, I am willing to implement it.