Comment 2 for bug 1841363

There is hard-code in nova.virt.libvirt.volumes.iscsi:get_config where `conf.driver_io=native`, which will result in a problem when we set `disk_cachemodes = block=writethrough` in /etc/nova/nova.conf.

In this case, when we try to attach a block device provided from cinder whose backend is LVM to a VM, the device cannot be successfully attached actually.