My server is running Ubuntu 12.04 LTS from /dev/sda3 (ext4). I experimented with CentOS 7 some time ago and that is on /dev/sda2 (xfs) and its grub is used for booting. It showed only the latest Ubuntu version in the grub menu, but that is fine.
I want the move from Ubuntu to CentOS to be smooth and first put some of the complicated services in docker containers.
Then I found docker runs only with vfs on the 3.16 So I downgraded to 3.13 As some recommended, rebooted in CentOS, ran update-grub, and rebooted. Now no entries for Ubuntu are shown.
I have restored the grub-menu from backup and can boot into Ubuntu again (but the newer kernel). How to have CentOS generate the Ubuntu kernel entries?