Is there an open source equivalent to piper, Google's version control tool?
Google stores all its codebase in a single repository called piper [1][2][3].
It has an approach that is very different than open source alternatives do (centralized 'cloud' service) and aims at scaling to a repository with billions of files, thousands of developers and millions of commits [1].
It doesn't seem Google open-sourced it nor plan to do so (contrary to their build system blaze and some other tools [4]).
Are you aware of any open source version control system with an approach similar to piper ?
[2] https://www.wired.com/2015/09/google-2-billion-lines-codeand-one-place/
[3] https://www.youtube.com/watch?v=W71BTkUbdqE
[4] https://opensource.google.com/projects/list/developer-tools?page=3
Answer
The short answer is no, it doesn't seem to exist.
As you can read in a Quora article, "it’s hard to tell where the version control system ends, and where some of the other parts of the development toolchain begin".
So, first, you need to be clear in what "features" you are interested in since you can be interested in a feature that is not Piper's responsibility.
Also, keep in mind that your server disk space and OS would limit the file count/size before the chosen VCS.
If you need a Centralized VCSs and billions of files, you could go with SVN or OpenCVS.
If you need a Distributed one with thousands of developers and millions of commits, take a look at Git, Bazaar, Bitbucket or Mercurial.
But do you really have all those requirements?
AFAIK there's no Piper's open source equivalent on the market.
In order to better understand Centralized and Distributed VCS, take a look at this Comparison between Centralized and Distributed Version Control Systems
Also, take a look at what is Google's repository like?