That's right, it should be noted however that the need of horizontally scaling the equivalent of the NameNode kicks in only when you really have a very large storage system, where large can be defined as:
"You know you have a large storage
system when you get paged at 1 AM because you only have a
few petabytes of storage left." [1]
Even if you are below that size you might have a large system, and even if using a single metadata master node might be a sound solution you still have lot of interesting problems to solve. Don't make systems more complex than necessary.
More pointers to google public material on colossus in [2].
Very few cluster sizes in enterprises reach this limit, and by limit I mean 1k -> 2k nodes. In reality, there's very little demand for namenode scalability at the current moment, but once this changes, the community will implement it.
"You know you have a large storage system when you get paged at 1 AM because you only have a few petabytes of storage left." [1]
Even if you are below that size you might have a large system, and even if using a single metadata master node might be a sound solution you still have lot of interesting problems to solve. Don't make systems more complex than necessary.
More pointers to google public material on colossus in [2].
[1] http://static.googleusercontent.com/media/research.google.co...
[2] http://www.highlyscalablesystems.com/3202/colossus-successor...