So let's say VM A is already running, and it's cloned to VM B. When that happens, we freeze the memory of VM A, and link it to VM B. For both VM A and VM B, any new write will be done to a new layer.

Thanks for replying.

So the logic is to check if VM A has a new fork. If yes, then start CoW to a new layer of blocks, and leave the current layer to be linked with VM B. If no, just don't use CoW.

I hope I got it right!

Yep, that's exactly it. So the moment the fork happens, we create both a new layer for VM A and VM B, so they can both use the same base layer.

Got it, thanks a lot!