People who use mongoperf to compare XFS and ext-X might get results that overstate the benefits of XFS. The problem is a per-inode mutex that is locked during writes with buffered IO so there won't be pending writes from pending threads. The mutex contention isn't as bad for reads but from past experience still hurts concurrent IO performance.

The workaround is to let mongoperf use multiple files. That would also make the mongoperf load more realistic given mongodb will use many files.

This is one example – http://edgystuff.tumblr.com/post/81219256714/tips-to-check-and-improve-your-io-performance-for-high