Comment 1 for bug 1788619

Have you tested this or just guessing that the libvirt driver in nova isn't doing the right thing? Because multiattach disks are always set to cache mode "none":

https://github.com/openstack/nova/blob/20bc0136d0665bafdcd379f19389a0a5ea7bf310/nova/virt/libvirt/driver.py#L423-L426

        # Shareable disks like for a multi-attach volume need to have the
        # driver cache disabled.
        if getattr(conf, 'shareable', False):
            conf.driver_cache = 'none'