Cannot Install Over Centos 7

I am trying to install Ubuntu 15.04 to replace my existing distro of Linux, Centos 7 Workstation. During the install I get to the point where it asks if I want Ubuntu to overwrite my existing disk (500gb hdd), in which I accept and hit continue, it then gets to the screen where it shows the drive onto which the os will be loaded, I click continue...and the it loops back to the previous page. I've using the manual partitioning program built into the installer and it shows all of my existing partitions in centos under dev/mapper/centos-home (approx 450 GB) and dev/mapper/centos-root (approx 50GB) and when I try to select each one, format it to EXT4 or FAT32, it will give me the option of creating a home partition and boot partition but no root, claims I need a root partition, and then sends me back to the previous "select which drive" page. Have never ever had a problem changing distribution of Linux until now. The computer I am putting this is on a Samsung U530 Ultrabook and I have already disabled UEFI in bios. Now when trying to load Centos I just get the emergency screen. Any help getting Ubuntu on this PC would be appreciated!

1 Answer

The /dev/mapper/{whatever} entries you describe are not partitions; they're logical volumes in a Linux Logical Volume Manager (LVM) configuration. Logical volumes are contained by one or more physical volumes, which are partitions. This extra level of indirection buys flexibility -- but the details of that would be a significant detour from an answer to your question.

If you use the "Something Else" installation option and plan to re-use your existing LVM configuration, you must explicitly assign at least two or three partitions or volumes, and possibly some others:

• /boot -- If you use LVM, /boot must be a partition and outside of the LVM. This is true of Ubuntu, CentOS, or any other Linux. Thus, there's probably already a /boot partition, with a device filename such as /dev/sda1, /dev/sda2, or something similar. This partition is normally pretty small -- typically in the 500MiB range.
• root (/) -- You must assign one partition or logical volume as your root (/) filesystem -- that is, its "mount point" is /. People who are unfamiliar with Linux partitioning often neglect to set a device as root (/) and then don't understand why the installation won't proceed. In an LVM configuration, this filesystem is usually a logical volume. Note also that root (/) is the base of the filesystem tree; it is not the same as /root, which is the superuser's home directory. The root (/) filesystem can be as small as a few gibibytes or as big as the whole disk, depending on your needs and preferences.
• swap -- Swap space is not strictly required, but most installations assign a partition or logical volume of one to two times the available RAM as swap space. In an LVM configuration, swap space is usually implemented as a logical volume.

Your CentOS installation probably has a /boot partition and at least the two logical volumes I've just described. It may have other logical volumes for /home, /var, and other purposes. If you want to re-use these existing partitions and logical volumes, you may, but it's up to you to figure out what partitions and logical volumes to use for what purposes. The old functions should be pretty obvious, unless there are numerous logical volumes.

As to the installer entering an endless loop if you try to use the option to overwrite existing partitions, that definitely sounds like a bug. I recommend you file a bug report. Bugs will only be fixed if developers know about them, and it's unlikely that the right developer will stumble upon your post here.

If you don't want to use an LVM configuration (Ubuntu doesn't use LVM by default), you should be able to work around the bug you've found by using any partitioning tool to delete the existing partitions, then re-run the Ubuntu installer. If you do this, you don't need to worry about the LVM setup's logical volumes; when you delete the containing physical volume partition, the logical volumes will disappear. You should reboot after deleting a physical volume, though, to be sure the kernel isn't still trying to use the logical volumes.