Google's proprietary method of storing search indexes on disk. It is a distributed file system used in Google's own datacenters, and it is embedded in the Google Search Appliance, which is as a self-contained search device. It is not available as software only.
Computer Clusters and Failsafe
The Google File System (GFS) is designed to run on multiple, standard x86-based servers. It stores data in large chunks that are replicated several times throughout Google's network, and it is designed to withstand any server failing entirely. Master servers store meta-data, while "Chunkservers" store the data files. See Google Search Appliance