Google is in the process of migrating their EXT2 file-systems over to the modern EXT4 file-system. This was brought up in a JFS benchmarking discussion. Google's Michael Rubin shared that they chose EXT4 after benchmarking it as well as XFS and JFS. Their results showed EXT4 and XFS performing close to one another, but with it being easier to upgrade from EXT2 to EXT4 rather than EXT2 to XFS, they went with the easier path. Btrfs is still too experimental for Google to even consider that an option at this point.