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qcow2 copy-on-write has nothing to do withfork(), which implements copy-on-write for RAM, not storage. See the wikipedia page, which explains it well, I think.

In the case of Nova instances, by default, a copy of the image will be made to the compute node where the instance runs. A qcow2 file is then created with the image copy as backing file; conceptually: qemu-img -f qcow2 -o backing_file=copy-of-image instancedisk.qcow2. The instance is the launched with instancedisk.qcow2 as root disk.

While conceptually similar, qcow2 copy-on-write has nothing to do withfork(), with fork(), which implements copy-on-write for RAM, not storage. See the wikipedia page, which explains it well, I think.

In the case of Nova instances, by default, a copy of the image will be made to the compute node where the instance runs. A qcow2 file is then created with the image copy as backing file; conceptually: qemu-img -f qcow2 -o backing_file=copy-of-image instancedisk.qcow2. The instance is the launched with instancedisk.qcow2 as root disk.