The Name Cache

Name cache is for recent (hot) filename lookups

Originally a global hash table, sized to the fixed number of vnodes

Not really suitable for a larger number of vnodes. 

Solution? the cache need not be global - when we do lookups, we know
the direcotry vnode - cache is now a RB trees (positive and negative)
in each directory vnode. 

Total size of cache still globally limited.

Scales to large numbers of vnodes and stays fast.