We are observing that after a blockcopy, the disk backend will inherit from original disk's cache mode configuration even though a new cache mode is given. Specifically, the function https://github.com/libvirt/libvirt/blob/cd94d891fb4b5cdda229f58b1dee261d5514082b/src/qemu/qemu_domain.c#L10887 which eventually calls  qemuDomainPrepareDiskSourceData and override input config.

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