[Ceph-users] Help pls (RBD Cache)

Hi there, Sorry for asking a question, which may be of very basic nature and asked many times before. But much Google search can not satisfy me. The question is about RBD Cache in write-back mode using KVM/libvirt. If we enable this, it uses local KVM Host's RAM as cache for VM's write requests. And KVM Host immediately responds to VM's OS that data has been written to Disk (Actually it is still not on OSD's yet). Then how can be it power failure safe? Is my understanding correct? If not, pls correct. This is very important for me. Thank you very much in advance. Best regards. Muhammad Junaid