GlusterFS is a free and open source distributed file system capable of scaling to several petabytes. Red Hat, Inc support it. It can easily handle thousands of clients and pretty common among the cloud computing or high profile web apps.
From the tutorial page:
I want to share my web server DocumentRoot /var/www/html/ across 2 Apache web server. Both of my web servers are behind load balanced reverse proxy powered by Nginx. How do I setup and install GlusterFS (distributed storage) which is a scalable network filesystem on Ubuntu Linux 16.04 LTS server? This tutorial shows you how to install GlusterFS on Ubuntu Linux 16.04 LTS server and configure 2 nodes high availability storage for your web server.