Google File System is "a scalable distributed file system for large distributed data-intensive applications". Initially used to store Google's search indexes and the crawling data, GFS is now mostly used to store user generated content. ACM has an interview with Sean Quinlan, who was a GFS tech lead and’s now a principal engineer at Google “Although organizations don't make a habit of exchanging file-system statistics, it's safe to assume that GFS is the largest file system in operation (in fact, that was probably true even before Google's acquisition of YouTube). Hence, even though the original architects of GFS felt they had provided adequately for at least a couple of orders of magnitude of growth, Google quickly zoomed right past that,” said Sean.
Source:→ Google OS