MongoDB 2.6 does not start on Ubuntu 15.10 and How To Install MongoDB In Ubuntu 15.10 Easily

systemd to upstart-sysv

     # ubuntu 15.04 advanced the default init system
     # from `upstart` to systemd

     # switch back to `upstart`
     $ sudo apt-get install upstart-sysv
     $ sudo reboot

install

     # 1. import public key
     $ sudo apt-key adv --keyserver keyserver.ubuntu.com --recv 7F0CEB10
     # 2. create a list file for mongodb
     $ sudo echo "deb http://repo.mongodb.org/apt/ubuntu trusty/mongodb-org/3.0 multiverse" | sudo tee /etc/apt/sources.list.d/mongodb-org-3.0.list
     # 3. reload local package database
     $ sudo apt-get update
     # 4. install
     sudo apt-get install -y mongodb-org

 
Adding the following lines preceding exit 0 in /etc/rc.local file

if test -f /sys/kernel/mm/transparent_hugepage/khugepaged/defrag; then
echo 0 > /sys/kernel/mm/transparent_hugepage/khugepaged/defrag
fi
if test -f /sys/kernel/mm/transparent_hugepage/defrag; then
echo never > /sys/kernel/mm/transparent_hugepage/defrag
fi
if test -f /sys/kernel/mm/transparent_hugepage/enabled; then
echo never > /sys/kernel/mm/transparent_hugepage/enabled
fi

    # 5. sudo reboot  

    # 6. mongo

service

     $ sudo service mongod start

if you want uninstall mongodb then ............... 

uninstall

     # 1. stop mongodb
     $ sudo service mongod stop
     # 2. remove package
     $ sudo apt-get purge mongodb-org*
     # 3. remove data directories
     $ sudo rm -r /var/log/mongodb
     $ sudo rm -r /var/lib/mongodb