Microsoft hasn't just become an enthusiastic open-source user; the company is now using the Git version control system to build Windows itself. The once prime example of proprietary software development is now relying on open source to create its trademark operating system. Who would have thought it?

Actually, you might have if you've been paying close attention. Back in 2013, Microsoft announced its roadmap for adding support for Git to its Visual Studio development-tool suite and Team Foundation app-lifecycle-management technologies. Later that same year, Microsoft Technical Fellow and TFS chief Brian Harry announced Microsoft would be backing Git as its distributed source-code-control platform.

Not everyone in Microsoft liked this idea, but as Harry blogged at the time, "the more we looked at it, the more it looked like the right thing to do".

As the years went by, Microsoft even made its own significant open-source contributions to Git. In 2017, Microsoft open-sourced Git Virtual File System (GVFS), under the MIT License. GVFS enabled Microsoft's product teams to scale the Git client to deal with its monstrously large source code repos.

Since then, Microsoft started porting all -- and I mean all -- the Windows code to Git and GVFS. The work is now largely done and Microsoft is enjoying the fruits of its open-source labor in creating the largest Git repo on the planet.

Harry wrote, "Over the past 3 months, we have largely completed the roll-out of Git/GVFS to the Windows team at Microsoft". That was no small job. "The Windows code base is approximately 3.5M files and, when checked in to a Git repo, results in a repo of about 300GB."

Read more: Microsoft uses open-source software to create Windows | ZDNet