On Fri, Oct 3, 2008 at 5:39 PM, Fong Vang <sudoyang at gmail.com> wrote: > So you're saying that the CentOS 4.x system is married with the 2.6.9 > kernel? Maybe the packaging of the kernel RPM is different between 4.x and > 5.x, but why would a 5.x kernel not work on a 4.x system, especially > considering you can always download the latest kernel from the kernel source > tree and run that so this doesn't sound right. If system call interfaces have changed, you *can't* always download and run the latest kernel from the kernel source tree. Sometimes programs that make system calls have to be recompiled along with the kernel. I haven't paid attention to whether there was any such change introduced frm 2.6.9 to 2.6.18; I'd sort of expect a bigger change in the version number if so. Aside from the binary compatibility issue. though, occasionally things like the layout of /proc and /dev change in incompatible ways. The introduction of udev between CentOS 3 and 4 makes it seriously problematic to run a CentOS 4 or 5 kernel on a CentOS 3 system, for example. You may have to install more than just the kernel.