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Pro Extremely low minimum system requirements
Puppy Linux requires 128MB of RAM and 333Mhz of CPU, although 256MB of RAM is recommended.
Pro The whole OS runs off of RAM
Since Puppy Linux is completely loaded and run off of RAM instead of HDD, it's read/write speed is not a factor.
Pro Designed to run as a live USB/CD
A portable version, that can be carried on a thumb drive without requiring installation, weighs less than 100MB; a Live CD - less than 150MB. You can even save your settings and files for Puppy on the external device/media.
Running off a live CD also has security benefits, as your system will reset to the known config after each boot.
Pro Linux distro designed for this exact purpose
Pro Offers a long term support release
Puppy Linux offers an LTS release called Precise.
Pro Different distro based versions available
Puppy Linux offers a few different releases.
The first is an Ubuntu based release called Tahrpup, by using this version the user is able to take advantage of all software and support from Ubuntu.
The second version is called Slacko, which is built upon the Slackware binaries.
The third is called Wary and it is built to support older hardware than the rest.
And the fourth is called Quirky, which is used as a base to explore new ideas.
Pro It can be the only distro that works
Sometimes, Puppy Linux is the only distro that works on an old machine: all other distros are more demanding.
Pro JWM used as window manager
JWM is a low resource window manager that is used in most Puppy Linux installs.
Con A bit ugly
Using puppy makes you feel like you just jumped 20 years into the past. Being that the purpose of Puppy Linux is to run or really low resource machines the software used is often on the older side, which is precisely why it has a 20 year old look.
Con Smallest software library ever
You can barely find any precompiled packages for Puppy.
Con No built in compositing
Being a low resource distro there is zero built in compositing for the JWM window manager. Though one can be added through a bit of work.
Con Not as light as it used to be
It's light, but not as light as it was 1-2 years ago.