Check memory usage in Linux

http://man.linuxde.net/free
http://man.linuxde.net/vmstat

https://www.cnblogs.com/kerrycode/p/6208257.html

https://en.wikipedia.org/wiki/Debian_version_history#Release_table

https://www.cnblogs.com/tsdxdx/p/7292149.html

https://www.debian.org/releases/stable/i386/release-notes/ch-upgrading.html

http://www.cnblogs.com/Security-Darren/p/4685629.html