Upgrading the system is one of the very common tasks a Linux Administrator usually do. One can easily update the system with Yum or apt-get commands but the main problem which everyone faces is to reboot the system after a kernel upgrade. This comes out to be a big problem for small organizations, where one don't have High Availability setup and can't afford to reboot the system because of the upgrade.
Since reboots are disruptive, many system administrators delay the update of there patches and makes their system vulnerable to attacks. Below given is the frequency of reboots needed in the last year for respective flavors of linux.