Google is in the process of upgrading their existing EXT2 filesystem to the new and improved EXT4 filesystem. Michael Rubin, who is in Google, shares that while EXT2 has been serving Google very well all these years, it has what is called "Read Inflation" - of which we will not go into details.
To bring Google's filesystem upto date, he added that Google has benchmarked three different filesystems - XFS, EXT4 and JFS. In their benchmarking, EXT4 and XFS performed, as impressively as each other. However, in view of the easier upgrade path from EXT2 to EXT4, Google has decided to go ahead with EXT4.
The storage stack of some of Google's most intensive applications have already been moved to EXT4. However, a real performance difference will not be seen until the whole migration to EXT4 has been completed.
You can read Michael Rubin's message in the link given below.