To add some context to my question, the majority of my experience is with Mac OS X. The default installation process in modern versions of Mac OS X, for volumes on which Mac OS X is already installed, is an "archive and install." Booting from installation media preserves data on the target volume while simply "over-installing" the operating system.
I am hoping to fully understand how Ubuntu 12.04 will handles re-installations under certain conditions: particularly where /var resides on a separate volume.
In my case, I have a 2 physical disks in a software RAID 1. On top of the RAID are 3 LVM partitions mounted at /, swap, and /var. My logic for placing /var on a separate volume is that this server will run libvert/KVM and the most critical data will be my VM's stored in /var. I wanted the ability to easily reinstall the OS without destroying my most critical data.
In the event that I damage my OS installation and need to recover, if I boot to the Ubuntu 12.04 live USB and simply run the installer without repartitioning or erasing any volumes, will /var be overwritten? Are only /boot overwritten? How does the re-installation process work?