Version 2.9.1
Rebalance

The WT_SESSION::rebalance method can be used to rebalance data sources' underlying btrees. If a tree has become unbalanced (that is, one part of the tree is excessively deep), WT_SESSION::rebalance rewrites the tree as a balanced tree.

The data source must be quiescent.

The WT_SESSION::rebalance method should never be needed, as WiredTiger btrees are maintained as balanced trees. It is only provided as a tool to handle the unexpected.