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Vm A also does copy on write. So VM B is still seeing the unmodified block.

Yes, and this is also the biggest challenge. Right now we use XFS to enable CoW, but that quickly leads to filesystem fragmentation. I'm still looking at a way that we can quickly let both VM A and VM B use the same base snapshot, and write new changes to either anonymous memory or a file.

Hi, not sure I understand.

What if VM A is a new VM? What happens to the block after copy-on-write? Just destroyed?

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!

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