[ryao]

Defragmenting a merkle tree required BPR, which temporarily breaks the structure intended to keep data safe. The only code known to have achieved it performed poorly and is behind closed doors at Oracle.

The benefits in terms of defragmentation are also limited because ZFS does a fair job of resisting fragmentation related performance penalties. The most that I would expect to see on a pool where poor performance is not caused by the best fit allocator would be a factor of two on sequential reads.