LizardFS allows storing data across several nodes (which run on commodity hardware, even inside virtual machines) and also has a built-in HA mechanism. This works very well if failover can be done in a manual or simple way (e.g. by using Pacemaker). Also there is an automated built-in failover mechanism if you are ready to spend money for a support contract (it is not expensive, though). Update: After having tested the commercial failover mechanism which comes with signing the support contract or a software demo I can say that it works as expected. It is easy to manage and just seems to work. To be fair I have to state that I didn't use it in production yet, only within tests. But it looks very good.
