Joyent VP for Engineering Bryan Cantrill colorfully details "Fork Yeah! The Rise and Development of illumos" in his presentation that answers the question "WTF is illumos?" in the following way:
( pdf of slides )
( 1 hr video )
WTF is illumos?
An open source descendant of OpenSolaris, which itself was a branch of Solaris Nevada which was the name of the release after Solaris 10, and was open but is now closed, and is itself a descendant of Solaris2.x, but it can all be called "SunOS5.x" -- but not "SunOS4.x" -- (that's different).
Starting in the summer of 2010, Garrett D'Amoreat of Nexenta -- with help from Rich Lowe, Jason King and others -- began the process of either writing the closed bits from scratch or porting them from BSD.
A complicated topic, as you can see, but Cantrill threads his way through the maze in an illuminating fashion, bearding the Oracle dragon in its for-profit den along the way.
illumos in the Cloud: What Is Joyent Up to?
Transcript of Garrett delivering illumos presentation
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