Namenode BlocksMap on Disk

[Dennis Kubes]

From time to time a message pops up on the mailing list about OOM 
errors for the namenode because of too many files.  Most recently there 
was a 1.7 million file installation that was failing.  I know the simple 
solution to this is to have a larger java heap for the namenode.  But 
the non-simple way would be to convert the BlocksMap for the NameNode to 
be stored on disk and then queried and updated for operations.  This 
would eliminate memory problems for large file installations but also 
might degrade performance slightly.  Questions:

1) Is there any current work to allow the namenode to store on disk 
versus is memory?  This could be a configurable option.

2) Besides possible slight degradation in performance, is there a reason 
why the BlocksMap shouldn't or couldn't be stored on disk?

I am willing to put forth the work to make this happen.  Just want to 
make sure I am not going down the wrong path to begin with.

Dennis