in the root CMakeLists.txt ...

if(WITH_KRBD AND WITHOUT_RBD)
message(FATAL_ERROR "Cannot have WITH_KRBD with WITH_RBD.")
endif()

considering the options being tested, should't that be

"Cannot have WITH_KRBD with WTIHOUT_RBD."

we want to have RBD enabled when Kernel RBD is enabled, isn't it ?

please correct me if my understanding is wrong

Milind