I found kernel is mismatch accidentally when I using "uname -r" to > check kernel version. So my question is what the harmness we will > get if I install a el7 rpm into a el6 system? > I would say it depends on the dependencies. If its just some userspace tooling then it will probably work ok. Its just x86_64 binaries after all. When you start getting into drivers and system level libraries then your probably gonna have a bad time but even then, the kernel doesn't change that much. Most of the new kernel stuff is for obscure virtualisation / containerisation techniques. Of course daemons will be a difficult area as the SystemD stuff might not there but it seems that there is still a lot of packages that ship with init.d scripts. I generally found el6 packages to be generally compatable with el7 and vice versa but there is no guarantee that the next update wont horribly break your system. In fact, it probably will :) Security was often a second concern to ' this system will horribly break if we update it so lets just turn off the internet!'