The original idea was weighted B-trees composed out of weighted rbyds,
with the two weight systems being independent. Descent down the B-tree
uses the same technique in the current metadata data-structure of
searching for which branch to take during fetch, basically getting the
search for free (well, on top of the already required O(m) fetch
operation).
But this is fundamentally flawed. While file names provide an absolute
reference for finding matches, weights are relative references. So we
don't have enough information to do weight-based lookup during fetch.
This smells just like the relative-vs-absolute key issues that led to
rbyd vs rbd trees in the first place...
One option is to do rbyd traversals at each B-tree node to build the
necessary information to figure out the weights. But with rbyd
traversals taking O(m log m), this makes B-tree lookups O(log n * m log m),
and B-tree traversals a messy O(n log n * m log m), which is acceptable, but
disapointing for what will likely be the most common operation in the
filesystem.
But the rbyd trees _are_ already weighted. A better solution might be to
go back and rethink the seperation of B-tree weights and rbyd ids.
Unfortunately, with only 16-bits available for rbyd ids, this would
likely require a rewrite of how rbyd tags are encoded...
Christopher Haster
cdc3a486d6