Thank you for your answers and sorry for not being clear about my problem.
Currently i have about 4TB of image files spread over 3 servers.
My application spreads these files randomly. On which server the file is located, is stored at the mysql database (replicated to each of these servers).
The image files are served by nginx and php directly from these servers (no proxy).
When one of these servers crashes, i have no failover and no redundancy.
Of course i can recover anything from backups, but 1 to 1.5TB of data on each server need much time to recover.
After a bit of research, i found MogileFS as the optimal solution.