The least-I/O method assigns each I/O to a path based on which path has the fewest outstanding I/Os. The number of I/Os is considered, but not the size of each I/O.
The least-blocks method assigns each I/O to a path based on which path has the fewest outstanding blocks. The aggregate size of the outstanding I/Os on each path, rather than just the number of I/Os, is considered.
These methods can provide more efficient use of bandwidth because they actually consider the utilization of each path before assigning each new I/O, unlike round-robin. However, the overhead of this calculation can drive up latency in some situations, so vendors typically provide round-robin as an alternative to least-I/O and least-blocks methods.